6:20 p.m. EST
New York's Governor Cuomo announces that the National Guard has deployed 230 soldiers to assist the NYPD and MTA with additional security.
5:55 p.m. EST
Boston University posted an announcement on their web site claiming that the third unidentified victim is a graduate student who was watching the race with two friends near the finish line. The student’s name may be released “pending permission from the family.”
5:45 p.m. EST
From the White House: “The President will travel to Boston [Thursday] 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."
5:25 p.m. EST
FBI agent Richard DesLauriers at an early evening press conference:"Regarding who might be suspected in this event, the investigation is in its infancy.... Some of our activity you might see, some of it you won't... At this time there are no claims of responsibility... the possibilities are wide open."
Press conference is currently live on CBS. Watch a live stream here.
Experts say they are skeptical that the attack was home-grown: “The bottom line is that no domestic extremist movement just based on their ideology alone would have a huge reason to attack the Boston Marathon."
4:50 p.m. EST
The Boston Globe's Shelley Murphy has more information on the exact location of the bombs. One was placed across the street from where Governor Deval Patrick had been sitting earlier.
Local police and firefighter unions are offering a $50,000 reward for information in the bombings.
3:20 p.m. EST
Investigators believe they have found the circuit board used to detonate the bombs.
2:50 p.m. EST
A Facebook event has been created to organize a walk that would complete the last five miles of the course.
In light of today's tragedy, let's remember, honor, and stand up for all those affected by the incident that occurred at the 117th Boston Marathon. We invite everyone to join us on Friday, April 19 at 4:30pm to walk from BC to Boston to stand united. For anyone who did not get to finish, For anyone who was injured, and For anyone who lost their life ... we will walk. We will walk to show that we decide when our marathon ends.
So far, 12,500 people have committed to attend.
2:45 p.m. EST
The New York Times is reporting that the explosives used in the blast consisted of a "pressure-cooker" device packed with pellets and sharp "nail-like" objects.
In a separate post, CNN says they have spoken with a "senior U.S. official" who says there is no indication that al-Qaeda is involved. Fox News editor Jon Passantino is reporting that the Saudi student whose apartment in Revere, MA was searched by police earlier today has been cleared as a suspect.
2:05 p.m. EST
Per The Boston Globe, the Saudi embassy spokesman Jail Aljubeir says the Revere, MA, man is not a suspect in the bombings and is "more a witness than anything else."
1:40 p.m. EST
The London Marathon will hold 30 seconds of silence at the start of Sunday's race, says ABC's Job Williams.
1:05 p.m. EST
A second victim has been identified as Krystle Campbell, a 29-year-old resident of Medford, MA.
Bill Richard, father of 8-year-old victim Martin Richard, has released a statement: "We thank our family and friends, those we know and those we have never met, for their thoughts and prayers."
12:20 p.m. EST
Reports are now coming in that the explosive were placed in 6-liter pressure cookers and placed in black duffel bags.
Doctors say they have been removing nails and ball bearings from the injured. It's likely they were part of the bomb.
Investigators are asking for anyone with photos of the Boston finish line to send them to firstname.lastname@example.org for analysis.
Boston's flag is flying over New York City Hall today.
Via Howard Wolfson/Twitter
11:50 a.m. EST
Police have executed a search warrant on an apartment in Revere, MA, just north of Boston. Officers were seen carrying several large bags from the house.
Updated casualty figures: 3 dead, 176 injured, with 17 in critical condition.
Officials told CBS News that the two bombs were disguised to look like discarded property.
CNN reporter Lisa Desjardins tweeted a photo of 8-year-old victim Martin Richard holding a sign at his school peace walk.
10:40 a.m. EST
We've posted a series of firsthand accounts from the handful of our writers that were competing.
4/16, 6:45 a.m. EST
The Boston Globe has identified the 8-year-old victim of the blasts as Martin Richard, the son of a runner from Dorchester, Massachusetts.
10:10 p.m. EST
A third death has been confirmed in the bombings.
8:15 p.m. EST
The Boston Globe is reporting that police are questioning a person of interest at Brigham and Women's hospital.
CNN has raised the number of injured being treated to 130.
ABC says that the explosions are believed to have been caused by "small portable devices."
Mass. General Hospital says that they are not on lockdown for a suspicious device, contrary to reports.
7:15 p.m. EST
NYT now reporting that a total of three unexploded devices were found by police.
7:00 p.m. EST
ABC News is reporting that the bombs were triggered remotely.
Gideon Gil, The Boston Globe's Health and Science editor, reported that 115 people are now being treated at 7 different Boston hospitals.
Reuters has a map graphic showing the marathon route and the exact location of the explosions.
6:45 p.m. EST
Bloomberg News:"White House says Boston Marathon explosions 'act of terror': official"
CBS's Alex Romano says a Saudi national is in fact in custody but denies involvement. Citing FBI source.
6:40 p.m. EST
NBC is now reporting that one of the two people killed was an 8-year-old.
6:25 p.m. EST
Differing reports on the state of cell service in Boston. Sprint spokeswoman Crystal Davis said, "Minus some mild call blocking on our Boston network due to increased traffic, our service is operating normally."
The third "explosion" in the JFK library was just a fire. It started in the mechanical room and is now out. All staff and personnel are accounted for.
6:15 p.m. EST
President Obama spoke for several minutes and did not answer questions: "We still do not know who did this, or why."
6:00 p.m. EST
President Obama will give a statement on the bombings at 6:10 p.m. EST. Watch a live stream here.
Spokesperson for the Boston Police say no one has been taken into custody: "At this time, we haven't been notified of any arrests or anyone apprehended."
FAA has ordered a No-Fly Zone over the site of the explosions.
5:45 p.m. EST
Fox 29 claims to have confirmed that a "person of interest" is in custody regarding the bombings. No mention of nationality.
5:40 p.m. EST
Outside's Bob Parks had just completed his first Boston Marathon when the first bomb detonated across the street. He said the explosion seemed to come from the side of a building. The force of it caused a temporary stampede.
New York Post say they have confirmed from "law-enforcement sources" that 12 are dead. They also say that authorities have identified a suspect—a Saudi national—and are keeping them under guard at an undisclosed Boston hospital. No other sources have reported this.
5:30 p.m. EST
Updated casualty figures: 2 dead, 90 wounded.
5:15 p.m. EST
Outside's Erin Beresini was reportedly stopped .3 miles from the finish line.
Boston Globe now reports more than 100 are being treated in area hospitals for injuries.
Boston Police are unsure if the third explosion at the JFK library is related. No injuries reported yet.
Boston.com has first-hand footage of one of the explosions. WARNING: Graphic content.
5:00 p.m. EST
The Boston Globe has tweeted revised casualty numbers: 2 dead, 64 wounded.
According to CBS, Police have suspended cellular service in the area, fearing a phone may be used to detonate a device.
4:50 p.m. EST
The Boston Globe has reported the exact locations of the two unexploded devices. One "on the glass footbridge over Huntington Ave near Copley place," the other "reported near the Harvard #MBTA station." Police have also cordoned off a block of Commonwealth Ave. Reports of a suspicious package.
Redditor Throwaway-apr15 has compiled a list of all the runners who were in the area within one minute of the explosion.
4:40 p.m. EST
Corresondent Whitney Dreier, who was entered in the race, is reporting from a hotel on lockdown. She says that 2 are dead and 28 injured. After the two initial explosions, police have detonated a third explosive device.
The Atlantic's photo blog In Focus is gathering images from the scene. WARNING: Very graphic.
Boston police are calling in all off-duty officers.
4/15, 2:30 p.m. EST
Reports are flooding in of multiple explosions near the finish line of the Boston Marathon. Two separate detonations were reported near the Boston library. There is no word yet on who or what is responsible.
Accounts on the number of injuries are varied, but the Boston Globe, who have a reporter on the scene, say dozens of people are injured.
BREAKING NEWS: At least dozens of people have been seriously injured, a Globe staffer on the scene says.
NECN anchor Jackie Bruno was also at the scene and witnessed the explosions.
I saw people's legs blown off. Horrific. Two explosions. Runners were coming in and saw unspeakable horror.
Stay tuned for more information on the ground.
UPDATE: Boston police have now confirmed that ther are casualties. No word on the cause.
Police in Boston confirm 2 explosions at 117th #bostonmarathon - confirm casualties. But no indication as to who/what might be responsible.
Via Tyler Wakstein/Twitter