FBI Agent Richard DesLauriers says the photos came from surveillance cameras, photos and other evidence near the explosion sites. One man is seen wearing a light-colored baseball cap, the other a dark cap. The man in the dark cap set down a backpack at the site of one of the blasts, DesLauriers said.
Within moments of the announcement, the FBI website crashed, perhaps because of a crush of visitors.
The images were released hours after President Barack Obama and first lady Michelle Obama attended an interfaith service at a Roman Catholic cathedral in Boston to remember the three people killed and more than 180 wounded in the twin blasts Monday at the marathon finish line.
The break in the investigation came just days after the attack that tore off limbs, shattered windows and raised the specter of another terrorist attack on U.S. soil.
The suspects are considered armed and extremely dangerous, DesLauriers said, and people who see them should not approach them.
"Do not take any action on your own," he warned.
Generally, law enforcement agencies release photos of suspects only as a last resort, when they need the public's help in identifying or capturing someone.
FBI / AP
Releasing photos can be a mixed bag: It can tip off a suspect and deny police the element of surprise. It can also trigger an avalanche of tips, forcing police to waste valuable time chasing them down.
Video and photos recovered in the investigation are being examined and enhanced by an FBI unit called the Operational Technologies Division, said Joe DiZinno, former director of the FBI lab in Quantico, Va.
Investigators are looking at video frame by frame – a laborious process, though one aided by far more sophisticated facial recognition technology than is commercially available, forensic specialists said.
"When you have something that is this high-profile, they are going to use every available resource that they have," said former Miami federal prosecutor Melissa Damian Visconti.