Investigators are seeking to talk to at least two men pictured in images taken before two bombs exploded at the Boston Marathon but there are no firm suspects yet, law enforcement and national security officials said on Thursday.
Authorities are examining thousands of pictures and video images taken by surveillance cameras, the media and ordinary citizens around the time of the blasts in a huge manhunt led by the Federal Bureau of Investigation.
Homeland Security Secretary Janet Napolitano said video from the scene of Monday's bombings near the marathon's crowded finishing line had caught the interest of investigators.
"We have been collecting video from a variety of sources," she told a hearing in Congress. "There is some video that has raised the question of those that the FBI would like to speak with. I wouldn't characterize them as suspects under the technical term, but we do need the public's help in locating these individuals," she said.
The New York Post published leaked photographs of two men in the marathon crowd which it said had been passed around among law enforcement officials. The men were carrying a duffel bag and a black backpack, which was missing in later photographs of the pair.
A law enforcement official warned against jumping to conclusions about possible suspects from the thousands of spectators at the marathon, and he and other officials ruled out the men pictured in the newspaper as persons of interest.
"Images of individuals related to this investigation have not been released by the FBI to the public," said the official, who declined to be identified due to the ongoing investigation. "Any images not released via FBI official channels should not be considered credible."
There is an intense debate inside the government as to whether such photographs should be officially released and the public should be asked for help identifying people in them, officials said.
For the moment, the government is holding back from formally releasing images, fearing a repeat of a fiasco which occurred when photos released following a bombing at the Atlanta Olympics in 1996, led to the arrest of security guard Richard Jewell, who turned out to be innocent.
Police are also chasing forensic leads produced by recovered pieces of Monday's bombs which killed 3 people and wounded 176. The devices were made with pressure cooker pots, pieces of which were recovered along with fragments of a black bag.
THOUSANDS OF PHOTOS
Access to thousands of photographs and videos might produce useful evidence but sifting through the sheer number of images is a huge task for police.
One of the officials said that the Boston bombings had created the "most photographed crime scene in the history of crime scenes." Other countries, most notably Britain, have historically used video surveillance pictures more extensively than U.S. criminal investigators.
At least two men already questioned by investigators, including a Saudi student who was burned in the explosions, have already been disregarded as persons of interest in the inquiry, one of the officials said. The second man was ruled out as a person of interest overnight Wednesday, he said.
Napolitano cautioned against expecting quick results.
"The investigation is proceeding apace. And this is not an NCIS episode," she said referring to a TV drama. " Sometimes you have to take time to put the chain together to identify the perpetrators. Everyone's committed to making sure that gets done," she said.