WASHINGTON - Massachusetts Governor Deval Patrick said on Tuesday "two and only two" explosive devices were found after Monday's bombings at Boston Marathon which killed three and injured at least 140.

Speaking at a briefing in Boston, Patrick confirmed there were no other unexploded devices that had been found and "only two" explosive devices went off near the finish line of Boston Marathon Monday afternoon.

FBI Special Agent Rick DesLauriers said at the briefing there were "no known additional threats" following the bombing at the Boston marathon.

Local police said it would take time to know "who did this" and encouraged the public to provide the authorities with "any video or photographs" that could be helpful clues to the investigation.

Local authorities said they were trying to open the locked-down area as soon as possible.

The twin bombs exploded at around 3 pm local time (1900 GMT) Monday within seconds of each other near the finish line of one of the world's most revered races.

President Barack Obama said Monday that the US government has not yet got "all answers" about the explosions, but vowed to hold anyone behind the bombings accountable.