The day after two bombs exploded near the finish line of the Boston Marathon, investigators began to unravel some of the details of what happened, and we began to learn about the lives of the three people who were killed.
FBI Special Agent in Charge Richard DesLauriers said that they believed the devices used in the attack may have been pressure-cooker bombs stuffed with BBs and nails. Investigators said the bombs may have been left inside nylon bags or backpacks.
"The investigation is in its infancy," DesLauriers warned. There have been no claims of responsibility and the range of possible suspects remains "wide open."
We updated this post through the day on Tuesday. (See this note about how we cover events such as this.)
Update at 6:50 a.m. ET, April 17: We're tracking Wednesday's developments here.
Update at 9:06 p.m. ET. Where Things Stand.
INVESTIGATION: FBI Special Agent in Charge Richard DesLauriers said during a televised press conference that investigators have recovered pieces of the bomb at the scene. DesLauriers said pieces of "black nylon, BBs and nails," which may have been inside a pressure-cooker style bomb, have been sent to Quantico, Va. for analysis.
DesLauriers confirmed that the bomb was placed in a black nylon bag or backpack but at this point, they have not determined the exact device stuffed inside of it.
NPR's Dina Temple-Raston reports that an official familiar with the investigation told her the bomb was "a pressure cooker design with nails and ball bearings inside."
The AP reports investigators "found pieces of an electronic circuit board possibly indicating a timer was used in the detonation of the bomb."
President Obama told reporters that it's clear this was an "act of terrorism," but that it's not known yet whether it was the act of an individual or a group, and whether it was foreign- or domestic-related.
DEATHS, INJURIES: Two victims, who died in the blast, have been identified: Martin Richard was 8 years old and Krystle Campbell was 29. A third victim is reported to be a Boston University graduate student.
Three people were killed and at least 176 were injured, according to Boston Police. At least 17 of the wounded were critically injured. Related post on the Shots blog: "Boston Doctors Compare Marathon Bomb Injuries To War Wounds."
SECURITY: As Boston and other major cities across the nation and around the world keep their police on alert, there's particular concern in London — which hosts former Prime Minister Margaret Thatcher's funeral on Wednesday and a major marathon next Sunday. "British officials on Tuesday urged a review of security measures," The New York Times says. Meanwhile, DesLauriers said Tuesday that there are "no known additional threats" in Boston.
A Boston University graduate student is the third person, who died the blast, the university said.
The student's name is not being released because they are waiting for permission from the family to do so.
The Associated Press quotes the Chinese Consulate in New York City, saying the student is a Chinese national.
Update at 7:25 p.m. ET. A Message Of Peace:
WBUR reports that there's a melancholy mood on the streets of Boston. WBUR Steve Brown came across saxophonist Kevin Busse, who told him he was there to "spread the message of peace." Listen:
Update at 7:09 p.m. ET. Boston Marathon Will Continue:
For the 118th time, the Boston Marathon will be run in 2014. That's according to a written statement from Thomas Grilk, the executive director of the Boston Athletic Association.
"Boston is strong. Boston is resilient. Boston is our home. And Boston has made us enormously proud in the past 24 hours. The Boston Marathon is a deeply held tradition – an integral part of the fabric and history of our community. We are committed to continuing that tradition with the running of the 118th Boston Marathon in 2014."
Update at 5:50 p.m. ET. President Obama Will Travel To Boston:
The White House just sent this press release:
"On Thursday morning, the President will travel to Boston to speak at an interfaith service dedicated to those who were gravely wounded or killed in Monday's bombing near the finish line of the Boston Marathon."
Update at 5:15 p.m. ET. Pieces Of Bomb Sent To Quantico:
FBI Special Agent in Charge Richard DesLauriers said during a televised press conference that investigators have recovered pieces of the bomb. DesLauriers said pieces of "black nylon, BBs and nails," which may have been inside a pressure-cooker style bomb, have been sent to Quantico, Va. for analysis.
DesLauriers confirmed that the bomb was placed in a black nylon bag but at this point, they have not determined what exactly what was in it.
"The investigation is in its infancy," said DesLauriers. There have been no claims of responsibility and the range of possible suspects remains "wide open."
DesLauriers asked the public to call in any tips to the FBI, which has set up a hotline at 1-800-CALL-FBI.
"Someone knows who did this," DesLauriers said.
Update at 4:27 p.m. ET. What Investigators Are Looking For:
Our colleague Liz Halloran talked to Bryan Cunningham, a former CIA officer, assistant U.S. attorney and deputy legal adviser for the National Security Council, about what investigators may be looking for at the moment.
A big focus is on trying identify the type of explosive. Cunningham said:
"Was it gun power or fertilizer like Timothy McVeigh used in the Oklahoma City bombing that would suggest a less-sophisticated device? Or plastic explosives that suggest you're dealing with someone better trained and with access to more sophisticated materials?
"If they can recover parts of the device, there may be a fingerprint, or some DNA. It's unlikely, but it happens. After the first  Trade Center attack in New York City, they pieced together enough of the truck that exploded to identify it."
Update at 4:18 p.m. ET. Runners Receive Their Medals:
WBUR reports that many runners who did not have a chance to pickup their belongings are doing so today. Many of them were also given finishers medals.
As you might expect, it was an emotional moment for many. WBUR reports:
"The runners would walk up to a police barricade that spanned the intersection and hand their bib number to a race volunteer. The volunteer would go to the assigned pile of yellow bags that were piled on the pavement and return the bag to the runner. They would also present the runner with a medal. Some volunteers just handed over the belongings and the medal, but one woman made a quiet ceremony out of the presentation by placing the ribbon around the runner's neck and giving each runner a hug.
"Even though most of the runners I saw finished the race — albeit diverted down Commonwealth Avenue — it was anything but a triumphant moment."
Update at 4:13 p.m. ET. Bombs Were In Duffle Bags:
Citing an official familiar with the investigation, NPR's Dina Temple-Raston reports that both bombs were of the pressure-cooker design. Both of them were in black duffle bags.
Update at 3:10 p.m. ET. 29-Year-Old Identified As Victim:
Multiple news organizations are reporting that 29-year-old Krystle Campbell, of Medford, Mass., was one of those killed by bombing.
The AP reports that her father said Krystle had "gone with her best friend to take a picture of the friend's boyfriend crossing the finish line on Monday afternoon."
Campbell said his daughter worked at a nearby restaurant and said she was a "very caring, very loving person, and was daddy's little girl."
NPR has not independently verified this death.
Quoting an official familiar with the investigation, NPR's Dina Temple-Raston reports that the man who's apartment was searched by the FBI is being treated as a witness not a suspect.
"He had burns that were a bit different from other victims and that was part of the reason for the 'abundance of caution' and a search. He is not under guard at the hospital and he is not under arrest," Dina tells us.
The official also tells Dina that the bomb was "a pressure cooker design with nails and ball bearings inside."
A "person briefed on the probe" tells The Associated Press that the bombs were "pressure cookers with metal [and] ball bearings" packed inside. It isn't know yet what was used to set off the explosions, the AP reports.
"A law enforcement source told CBS News that one of the explosive devices appears to have been placed in a metal pressure cooker (a metal kitchen pot with a locked down top) which had been placed in a black nylon bag or backpack. Investigators also found pieces of an electronic circuit board possibly indicating a timer was used in the detonation of the bomb.
"A law enforcement official told CBS News that the two bombs that exploded were made to look like discarded property. It is still unknown if one or both bombs were in garbage cans. One may have been on the sidewalk."
According to a 2004 Department of Homeland Security memo, "these bombs are made by placing TNT or other explosives in a pressure cooker and attaching a blasting cap at the top of the pressure cooker."
NPR has not independently confirmed that authorities suspect pressure cooker bombs were used.
As we said when the day began: We will focus on news being reported by NPR, other news outlets with expertise and statements from authorities in position to know what's going on. And if some of that information turns out to be wrong, we'll update.
Speaking at the White House, the president just said that "given what we now know about what took place, the FBI is investigating it as an act of terrorism." What isn't known, he said, is whether it was the act of an individual or a group, and whether it was foreign- or domestic-related.
The way people at the marathon responded and helped each other, the president added, shows that "the American people refuse to be terrorized" even by such a "heinous and cowardly act."
Update at 11:15 a.m. ET. Obama To Speak:
The White House says President Obama plans to deliver a statement to the nation at 11:30 a.m. ET.
Update at 10:50 a.m. ET. A Heartbreaking Tale.
At a news briefing now underway, Boston Police Chief Ed Davis just updated the number of people injured. He says there were "176 casualties that presented at hospitals."
FBI Special Agent in Charge Rick DesLauriers then tells reporters that "we will go to the ends of the earth" to apprehend those responsible.
Update at 10:02 a.m. ET. No Comment On Anyone Being In Custody:
Asked if any suspects are in custody, FBI Special Agent in Charge Rick DesLauriers tells reporters he's "not going to say who might or might not be in custody right now."
Update at 10 a.m. ET. "Act Of Cowardice":
The bombings were an "act of cowardice," Suffolk County (Mass.) District Attorney Dan Conley just told reporters. He promised that "justice will be served."
There are "no known additional threats," FBI Special Agent in Charge Rick DesLauriers just told reporters in Boston. He also said the investigation will take some time and asked for the public to continue calling in tips and providing photos or videos taken at the scene.
Update at 9:45 a.m. ET. No Additional Bombs Found:
Authorities are now briefing reporters. Massachusetts Gov. Deval Patrick (D) just said that "all ... parcels have been examined" and no additional bombs were discovered at the scene. He said it should be emphasized that "two and only two explosive devices" were involved in the attack.
Update at 9:10 a.m. ET. BBs May Have Been Packed In Bombs:
"Investigators refused to give any specifics on the bombs and say, for example, where they might have been hidden or whether they were packed with shrapnel for maximum carnage, as is often the case in terror bombings overseas," The Associated Press writes. "But Dr. Stephen Epstein of the emergency medicine department at Beth Israel Deaconess Medical Center said he saw an X-ray of one victim's leg that had 'what appears to be small, uniform, round objects throughout it — similar in the appearance to BBs.' He said it remained to be determined what exactly the objects were."
Update at 8:50 a.m. ET. "Active And Fluid" Investigation:
The Boston Globe writes that "officials last night called the investigation 'very active and fluid.' Authorities were talking to at least one person at Brigham and Women's Hospital, according to sources familiar with the questioning."
Update at 7:45 a.m. ET. On The Young Victim:
"A beloved Dorchester family watching the Boston Marathon near the finish line in Copley Square was caught in one of two bomb blasts that exploded on Monday afternoon," the Dorchester Reporter writes. "Martin Richard, 8, was killed in the explosion, while his sister Jane, 7, suffered a grievous injury to her leg. Their mom, Denise, was also badly hurt in the blast and underwent emergency surgery yesterday to save her life."
As we reported earlier, authorities served a search warrant on a home in the Boston suburb of Revere late Monday, and investigators left with some bags. On its Facebook page, the Revere Fire Department reports that:
"Deputy Chief, Engine 5, Ladder 1, Cataldo Paramedics & Chief were called in for Operational standby for State Police Bomb Techs for search of a 'person of interest' apartment at 364 Ocean Ave. FBI, ATF, ICE, Boston PD, Revere PD as well as MSP detectives and Bomb Techs from Boston PD and State."
Update at 6:40 a.m. ET. Authorities Seeking Photos.
"Good police work," tweets The New York Times' Nicholas Kristof, who is in Boston. "At Boston airport, official is asking every person in security line if they have photos from scene."
6:25 a.m. ET. FBI Takes Charge; Estimate Of Number Injured Rises:
On the morning after explosions ripped through an area packed with runners, spectators, race officials, medical personnel and others at the finish line of the Boston Marathon, the FBI is heading up the investigation into who was responsible.
The Associated Press reports that the FBI "has served a search warrant on a home in suburban Boston in the investigation into the Boston Marathon bombing attack. Massachusetts State Police confirm that a search warrant related to the investigation was served Monday night in Revere, but they haven't said anything else. Some investigators were seen leaving the house early this morning, carrying brown paper bags, plastic trash bags and a duffel bag."
NPR's Dina Temple-Raston reports for Morning Edition that investigators are being very cautious about what they reveal and are cautioning that it may be some time before they can say with any certainty who they think was behind the attack. As she's previously reported, they don't want a repeat of what happened after the bombing at the 1996 Summer Olympics in Atlanta, when security guard Richard Jewell was mistakenly blamed. It took years for investigators to determine that the real bomber was Eric Rudolph, an anti-government, anti-abortion serial bomber.
Along with identifying who was responsible, authorities will also be pushing to determine whether the attacks were foreign- or domestic-related.
The investigation will be aided by the hundreds, if not thousands, of potential witnesses and the large number of videos that were being shot, by professionals and spectators, at the time of the explosions. But the scene is also extremely complicated. Abandoned backpacks and other gear left behind by runners and spectators litter the area. Those have to be searched not only for clues, but for explosives.
As of early Tuesday, authorities said they had not yet taken anyone into custody either as a "person of interest" or a suspect.
Among the other early Tuesday reports:
-- The 8-year-old victim was from nearby Dorchester, Mass., according to the Boston Globe.
-- One person could "easily" have been behind the attacks, retired FBI bomb technician Kevin G. Miles tells the Boston Herald.
-- Boston police plan a 9:30 a.m. ET news conference.
Our colleagues at WBUR have also resumed their live blogging this morning. They report that "overnight the number of injured rose. At last count, at least 151 are being treated at nearly a dozen hospitals around Boston."
Note: As happens when stories such as this are developing, there will likely be reports that turn out to be mistaken. Monday, for example, authorities at one point said they thought there had also been an explosion at the JFK Library in Boston. But it turned out there had been a fire, not an explosion, and there's no known link at this time to the marathon attacks.
We will focus on news being reported by NPR, other news outlets with expertise and statements from authorities in position to know what's going on. And if some of that information turns out to be wrong, we'll update.