Google has mobilized the social networks in setting up a Boston Marathon Person Finder page to help family and friends connect with beloved ones in the city after the deadly bomb blasts.

The Person Finder tool can be also used for finding any kind of information about them and about developments in Boston.

There are two main buttons: I'm looking for someone OR I have information about someone.  The Boston Marathon explosion finder is already tracking thousands of people.

In the hours after the blasts, the word "attacks" was the most mentioned on Facebook and Twitter.

The death toll in the explosions has risen to three while more than 100 are reported injured, some seriously.

TV footage showed bloodied runners and spectators being treated at the scene and the road strewn with debris.

The FBI has said that they were looking at a "potential terrorist investigation," but it's not clear whether the origin was domestic or foreign. However, President, Barack Obama, has been more cautious in speaking of terrorism, stating in a TV address: "we will find out who did this" and that those responsible would feel the "full weight of justice."

Huge crowds, including thousands of foreign visitors, were in the area watching the runners take part in the world's oldest annual marathon.

It was also Patriots Day, commemorating the opening battle of the Revolutionary War.

The explosions occurred at about 2:45 p.m. Eastern Time, more than two hours after the first of the race's nearly 27 000 runners had crossed the finish line.