Washington, Apr 19 (IANS): Describing them as possibly armed and "extremely dangerous", the Federal Bureau of Investigation (FBI) has released pictures and videos of two men with a direct appeal to the public to help them find the prime suspects in the Boston Marathon bombing that killed three people and wounded about 180.

The men were photographed walking down Boylston Street, one behind the other, near the marathon finish line where two near-simultaneous blasts went off at 2.50 p.m. Monday, killing three people and wounding about 180.

"Somebody out there knows these individuals as friends, neighbours, co-workers or family members of the suspects," Special Agent Rick DesLauriers, the head of the FBI's Boston office, said at a media briefing in Boston Thursday.

"And though it may be difficult, the nation is counting on those with information to come forward and provide it to us," he said even as he cautioned that the men should be considered armed and "extremely dangerous".

"No one should approach them. No one should attempt to apprehend them except law enforcement," DesLauriers said.

In particular, FBI asked for help from anyone standing in front of the Forum restaurant, where the second bombing happened.

One video, which officials said they did not release, shows the two men walking slowly away after a bomb exploded while the crowd fled.

"Suspect 1" was seen wearing a light-coloured, collarless shirt underneath a dark-coloured jacket and wearing a dark baseball cap.

"Suspect 2" wearing a light-coloured hooded sweatshirt, a black jacket and a white baseball cap turned backward was seen setting down a backpack at the site of the second explosion "within minutes" of the blasts.

DesLauriers said the FBI was seeking additional images captured by customers at the Forum restaurant, the site of the second blast, where the suspect in the white cap put down his backpack. "No bit of information is too small," he said.

Law enforcement agencies have received 3,000 images in addition to surveillance videos and TV news footage, which added up to several terabytes of data, investigators said.

The images were plugged into facial recognition software programmes and pored over by a team that includes more than 1,000 Massachusetts and federal law enforcement agents.

Within the last two days, investigators identified one "person of interest" in the case, DesLauriers said. They went back over the images and data to determine whether that person was alone or with others, leading to the identification of the second suspect.

The FBI has also found what its investigators believe to be the detonation system for the bombs - a circuit board and parts from a toy remote control vehicle, a counter-terrorism official said. Investigators have also contacted the maker of a battery found in the debris of the blasts.

Meanwhile, donning the role of the healer, President Barack Obama Thursday visited Boston as the city rallied at an inter-faith service dedicated to the victims.

American people refuse to be terrorised and whoever is behind the bombings will be caught, he said, declaring "Yes, we will find you. And yes, you will face justice."

Obama also stopped at a high school to thank a group of first responders and volunteers and met with patients recovering from the attacks at Massachusetts General Hospital.

The first lady Michelle Obama met patients, families and hospital staff at Boston Children's Hospital and Brigham and Women's Hospital, the White House said.