"One of the department stores down on Boylston St. submitted a video system which has confirmed that a suspect is seen dropping a bag near the point of the second explosion and heading off, and there's been a confirmation of a description of someone leaving the scene," Stephen Murphy, the Boston City Council president, told the Associated Press.

"I know it's very active and very fluid right now, they're on the chase, they may be on the verge of arresting someone."

Rep. Bill Keating (D-Mass.) said Wednesday he has heard from a well-informed congressional source that the FBI has video of a person dropping a bag carrying a bomb into a garbage can. The former district attorney said the videos are providing "a treasure trove of information."

"If you have the image of a person [that put the bag in], that's big," he said.

Contradictory reports of whether federal investigators had successfully identified or apprehended the suspect dominated the afternoon's media coverage, with CNN and the Associated Press reporting and then retracting that a suspect had been apprehended.

Boston police and the FBI later issued statements clarifying that no one was in custody. Rep. Mike McCaul (R-Texas), who chairs the House's Homeland Security committee, said he also had been told that no arrest was made.

Calling many reports in the immediate aftermath of the bombings "inaccurate," the FBI issued a statement imploring the media to exercise caution.

"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 channels before reporting," the FBI said in a statement.

Gov. Deval Patrick, speaking to CNN, refused to confirm that a suspect had been identified from the tapes, saying he had been briefed on the latest findings of the investigation but received that information "confidentially."

Patrick said that while authorities were "making progress," there was a lot of "misinformation flying around."

"It's going to be slow, it's going to be methodical," he said.

The FBI was originally scheduled to brief reporters about updates on the case at 5:00 p.m., but that news conference was delayed after an apparent bomb threat at Boston's federal courthouse. Journalists on the ground in Boston reported that the federal courthouse had been evacuated Wednesday afternoon. Fire trucks and K-9 units were observed outside the area, and staff could be seen leaving the building.

NBC News reported that the press conference would be rescheduled for the 8:00 hour.

White House press secretary Jay Carney said investigators "do not know at this time yet whether it was an organization or an individual, foreign or domestic."

"But this investigation is now not even 48 hours old, and it is important that we maintain the integrity of the investigation," Carney said. "It is important, as both state and local law enforcement officials and government officials, as well as federal officials, have made clear that the American people provide whatever information they might have that could be of assistance in this investigation."

--This report was updated at 7:10 p.m.