Video footage has pointed to a possible suspect in the Boston bombings.

Investigators are examining CCTV footage of a man they believe planted one of two bombs at the finish line of the Boston marathon on Monday.

It's understood the man may have dropped a black bag at one of the bomb sites.

The confirmation of the video is the first sign that the FBI may be closing in on whoever was behind the attacks, in which 3 people died and 170 were injured.

The explosions at the Boston Marathon finish line has been described as the worst attack on U.S. soil since September 11, 2001.

The US President, Barak Obama, will visit Boston later to remember the victims of the city's marathon bombings.

The bombings, as well as subsequent reports that someone tried to mail the deadly poison ricin to President Barack Obama - the second report of such a letter in two days - have created a climate of uncertainty in the country.

In Washington, authorities were investigating a letter addressed to the president after the contents preliminarily tested positive for ricin. Authorities had intercepted a similar letter sent to Republican Senator Roger Wicker of Mississippi on Tuesday.

The FBI said late yestersay it had arrested Paul Kevin Curtis, of Corinth, Mississippi, in connection with the letters.

Earlier the agency had said there was no indication of a connection between the ricin letters and the Boston bomb attacks.