Washington, Apr 18 (IANS): In a major breakthrough, investigators have zeroed in on two men as "possible suspects" who were seen in security video footage and other images of Boston Marathon moments before the deadly twin blasts.

A circular sent out Wednesday to federal and state agencies features the photos "in an attempt to identify the individuals," who were described as being of "high interest" to investigators hunting for the Boston bombers.

The bombings, described by President Barack Obama, as "an act of terror", killed three and injured about 180 others in the first major terrorist attack on US soil since the September 11, 2001 terror attacks.

One of the men is seen carrying a black backpack, according to CNN. An FBI official earlier said that authorities believe the bombs were placed inside a black nylon backpack or bag.

The channel citing an unnamed source said that authorities had not yet identified the two men by name and that the photographs were not being released to the public for fear of impeding the investigation.

The New York Times citing a senior law enforcement official said the videos also showed at least a handful of others whom the authorities want to question, either because of what they appear to be doing in the video or their proximity to the blasts.

The authorities were trying to boil down the number of people of interest in the videos and would then decide whether to ask the public's help in locating them, he said.

"It's a crowd, there are a lot of different angles. It is not like some television-produced video - there's a lot that isn't clear," the official was quoted as saying. "But most interpretations support the notion that one man is seen dropping a bag."

Earlier Wednesday, CNN said two official sources with knowledge of the investigation had identified a man-who also hasn't been named-as a possible suspect in the attack.

Seen on a video, this man wore a white baseball cap. One of the sources cited by the channel said the cap was on backwards and the man was also wearing a light-coloured hooded sweatshirt and a black jacket.

It was not immediately known if this man is one of those alluded to in the photographs distributed to law enforcement officials.

Authorities claimed to have had made "significant progress" in the case but no arrests have been made yet.