"We can confirm that all of the remaining runners who were out on the course when the tragic events unfolded have been returned to a community meeting area.
"At this time, runners bags in Boston which remain unclaimed may be picked up by runners presenting their bib number or proof of race participation at the Castle, at 101 Arlington Street, in Boston.
"At this time, we are cooperating with the City of Boston, the Commonwealth of Massachusetts, and all federal law enforcement officials.
"We would like to thank the countless people from around the world who have reached out to support us today."
[Update, 7:57 p.m. ET]
Doctors are "pulling ball bearings out of people in the emergency room," a terrorism expert briefed on the investigation told CNN's Deborah Feyerick.
The same source said the blasts resulted in at least 10 lost limbs.
[Update, 7:43 p.m. ET]
An 8-year-old boy was among those killed, a state law enforcement source said, according to CNN's John King.
[Update, 7:38 p.m. ET]
At least 132 people -- including eight children -- have been injured in the bombings, according to Boston-area hospitals. Boston police earlier said that two people were killed.
At least 17 of the injured are in critical condition, and at least 25 are in serious condition, area hospitals said.
[Update, 7:08 p.m. ET]
A witness, Marilyn Miller, told CNN that she was about 30 feet away from the first bomb when it went off. The second bomb came about 12 seconds after and about 50 to 100 yards away from the first, according to authorities and an analysis of video from the site.
Miller was waiting for a runner who, it turns out, was probably about 10 minutes away from the finish line.
"We saw injuries all around us," Miller said. Someone was putting pressure on a woman's neck. "A little boy, his leg was torn up. A woman, (people) were (shouting), 'Critical, critical, get out of out way!'"
[Update, 6:51 p.m. ET]
At least 110 people have been injured in the bombings, according to Boston-area hospitals.
[Update, 6:49 p.m. ET]
Boston cell phone services were overloaded in the wake of the blast, slowing the city's network dramatically and hampering the investigation in the early going, federal law enforcement officials told CNN.
Unconfirmed rumors began circulating on social media and elsewhere that law enforcement had shut down cell service to prevent more explosives from being detonated remotely. But mobile companies were saying that was never the case, CNN's Doug Gross reports.
"Verizon Wireless has not been asked by any government agency to turn down its wireless service," a spokesman for that company told CNN. "Any reports to that effect are inaccurate."
In other media reports, Sprint similarly denied being asked to shut down service.
Online, Bostonians were being encouraged to stay off of their mobile phones except for emergencies and even open up their wireless connections to help take the load off of the cellular data network.
"If you live or run a business in #Boston near bombsite (please) open your wifi for people to use," tweeted Disaster Tech Lab, an Irish nonprofit dedicated to providing technology to assist in emergency situations.
[Update, 6:47 p.m. ET]
Initial tests indicate that the two bombs were small and possibly crude, with the tests not indicating any high-grade explosive material was used, a federal law enforcement official with knowledge of the investigation told CNN national security contributor and former homeland security adviser Fran Townsend.
The source said the FBI considers the incident a terrorist attack, "but they've made clear to me they do not know at this time whether those responsible for the attack were a foreign or domestic group," Townsend said.
[Update, 6:35 p.m. ET]
U.S. Rep. Bill Keating, D-Massachusetts, said an unexploded device was found at a hotel on Boylston Street, and another unexploded device was found at an undisclosed location.
Keating, who is a member of the House Homeland Security committee and has spoken to law enforcement sources, tells CNN's Dierdre Walsh that the incidents were a "sophisticated, coordinated, planned attack."
[Update, 6:14 p.m. ET]
More from President Obama, who just wrapped up his brief statement at the White House: "We still do not know who did this or why ... but make no mistake: We will get to the bottom of (this). We will find out who did this. We will find out why they did this. ... Any responsible groups will feel the full weight of justice."
[Update, 6:11 p.m. ET]
President Barack Obama is speaking about the bombings now: "The American people will say a prayer for Boston tonight, and Michelle and I send out deepest thoughts and prayers to the victims," Obama said at the White House.
[Update, 5:59 p.m. ET]
A witness who is a doctor in his 50s was about 200 feet away from the finish line near the Prudential building when the bombings occurred. He felt the blast to the point that it made him and others around him jump in the air, and some others around him fell down on the ground, he said, according to CNN's Eden Pontz.
The doctor said he heard two blasts about five seconds apart. He said there was confusion all around him, and he was hustled into the nearby Mandarin Hotel. Officials wouldn't let them leave the hotel for a bit, and he says all who were there were all frisked by police. He said that when he left, he saw broken storefronts and lots of blood.
[Update, 5:51 p.m. ET]
President Barack Obama is expected to deliver a statement at about 6:10 p.m. ET from the White House.