This is a breaking news story. Check back for updates.
Update 3:51 p.m.: The courthouse was evacuated because of a large media presence at the scene, not because of a bomb threat, according to the Boston Globe's Maria Cramer:
Source telling me that Fed Courthouse evacuated b/c of my brethren in the media. We apparently created a false alarm.
â€” Maria Cramer (@GlobeMCramer) April 17, 2013
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Update 3:24 p.m.: The Boston federal courthouse has been evacuated and bomb-sniffing dogs have arrived at the scene.
Hundreds of people descended on the courthouse amid reports that a suspect was in custody and was expected to be taken to Boston federal court. Those reports later proved to be inaccurate.
An officer responding to the scene told the Washington Post that the evacuation was prompted by a bomb scare, but that report has not been verified.
Update 3:03 p.m.: Investigators are exploring the possibility that more than one person committed the bombings, the Los Angeles Times reports.
Citing a federal official, the paper said video footage or a photo of the scene depicted "two men with two backpacks."
Update 3:00 p.m.: The FBI is reiterating that no arrests have been made in connection with the Boston explosions, and derided media outlets that initially reported on an arrest.
"Contrary to widespread reporting, no arrest has been made in connection with the Boston Marathon attack. Over the past day and a half, there have been a number of press reports based on information from unofficial sources that has been inaccurate," the bureau said in a statement. "Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting."
Update 2:47 p.m.: A possible suspect in the bombings has been described as a white man who was wearing a white backwards baseball cap, a gray hoodie and a black jacket, CBS reports.
The suspect was captured on video placing a bag at the site of the second explosion outside the Forum restaurant on Boylston Street in Boston, according to the network's Bob Orr. The man is then seen leaving the area before the bomb detonated.
Update 2:37 p.m.: The Boston Police Department says reports that an arrest has been made are inaccurate. CNN and the Associated Press originally reported that a suspect was arrested; CNN has withdrawn its report, but the AP has yet to back down from its report.
Despite reports to the contrary, there has not been an arrest in the Marathon attack.
â€” Boston Police Dept. (@Boston_Police) April 17, 2013
First Lady will be going to Boston
Here's the scene outside Boston federal court:
Update 2:07 p.m.: Multiple news sources are reporting that the suspect is in custody. The Associated Press reports that the suspect is expected to be taken to a federal courthouse in Boston.
BREAKING: Law enforcement official: Boston Marathon bomb suspect in custody, expected in federal court. -BW
â€” The Associated Press (@AP) April 17, 2013
Update 2:00 p.m.: The suspect in the Boston explosions is expected to be taken into custody by U.S. Marshals and then taken to a Boston courthouse, according to the AP. Officials are expected to give a briefing at 5 p.m. EDT.
An arrest in the Boston Marathon bombings is either imminent or has already taken place, according to multiple sources.
A law enforcement official told the Associated Press that the arrest is "imminent," while CNN is reporting that an arrest has been made. Boston television station WCVB reported at 1:30 p.m. EDT that the arrest has either already taken place or is imminent.
"Official says bombing suspect is to be taken into custody and taken to a Boston courthouse," the AP tweeted.
MORE: Official says bombing suspect is to be taken into custody and taken to a Boston courthouse: apne.ws/11hIJkO -BW
— The Associated Press (@AP) April 17, 2013
Video surveillance from a Lord & Taylor store near the explosions proved helpful in identifying the possible suspect, according to CNN.
The bombs detonated around 2:50 p.m. Monday near the finish line of the Boston Marathon, killing three people -- including an 8-year-old boy -- and injuring more than 170 others.
The 8-year-old boy, Martin Richard, of Boston, along with 29-year-old restaurant manager Krystle Campbell of Arlington, Mass., and Boston University graduate student Lu Lingzi of China, were identified as the three people killed.
Boston Mayor Thomas Menino said the city will pull through the attacks.
"This is a bad day for Boston, but I think if we pull through together we'll get through it," he said at a news conference Monday shortly after the explosions.
The breakthrough in the investigation came about 48 hours after the blasts. Video provided to the FBI from a Boston television station was also said to be helpful to the case.