The FBI on Thursday released photos and a video of two men it called suspects in the deadly bombings at the Boston Marathon this week and asked for public help in identifying them.
According to CNN, the two men were photographed walking together near the finish line of the marathon before the explosions that killed three people and wounded about 180.
One of the two men was seen setting down a backpack at the site of the second explosion, said Special Agent Rick DesLauriers, the head of the FBI's Boston office. DesLauriers said the men should be considered armed and "extremely dangerous."
The FBI's website crashed in the minutes after the pictures and video footage were released, AFP reported.
The FBI said it had no details of the identities or origin of the two men, who were only named as Suspect One and Suspect Two. Both appeared to be young men, one dressed in a white baseball cap and one in a black cap.
Earlier, U.S. President Barack Obama vowed that the "evil" Boston marathon bombers would be found.
"Yes, we will find you, and yes, you will face justice," Obama told a special service in the city.
"We will find you, we will hold you accountable," he told a congregation of 2,000 in a special visit to show national solidarity after Monday's attack, according to AFP.
"If they sought to intimidate us, to terrorize us," Obama said, then "it should be pretty clear by now that they picked the wrong city to do it."
Obama, who also met relatives of the dead and recovering victims at Massachusetts General Hospital, said Americans had seen "the face of evil" in the attacks.
Obama has vowed a relentless hunt for the attackers and authorities say they want to speak to individuals seen in surveillance camera images around the marathon finish line that was devastated by the pressure cooker bombs.
"There is some video that has raised the question of those that the FBI would like to speak with," Homeland Security Secretary Janet Napolitano told a Congressional hearing, according to AFP. But she added that she would not call the "individuals" in the images suspects.
No claim of responsibility and no arrests have been made in connection with the worst attack in the United States since the September 11, 2001 atrocities.