In pictures: Boston Marathon explosions

FBI agents at scene of Boston blasts. 15 April 2013

The FBI is leading the investigation into Monday's twin blasts at the Boston Marathon. Special Agent Richard DesLauriers described it is as "potential terrorist investigation".

Medical workers on the scene in Boston, 15 April 2013

At least three people died and more than 100 were injured - some critically - in the explosions close to the finish line of the annual marathon.

Competitors run as the first explosion is seen near the finish line of the marathon, Boston, 15 April 2013

This image from NBC shows the first blast, with runners and spectators approaching the finish line.

Second blast at the Boston Marathon

The second explosion came as people rushed to help those caught in the first.

Runners continue to run towards the finish line of the Boston Marathon as an explosion erupts near the finish line of the race

A photographer captured the second explosion as runners made their way to the finishing line.

The finish line of the Boston Marathon shrouded in smoke, 15 April 2013

Runner Mike Mitchell of Vancouver, Canada, said he had just finished the race when he looked back at the finish line and saw a "massive explosion".

Medical workers take injured people away from the scene, 15 April 2013

The incident came at around 14:50 local time (18:50 GMT) - about three hours after the winners crossed the line.

A man walks away with minor injuries from the Boston marathon bombings, 15 April 2013

Some were able to walk away from the scene of the explosions.

Runners at Boston Marathon, 15 April 2013

What had been a scene of celebration was suddenly shattered. Runners were left reeling in shock and despair.

Blood on the streets at the scene of one of the explosions at the Boston marathon, 15 April 2013

Those seriously injured in the explosions were quickly taken to hospital, leaving a scene of carnage behind.

State police near Kenmore Square, Boston, 15 April 2013

The cause of the explosion has not been confirmed but the White House says it is in contact with state and local authorities. Emergency services are on high alert and state police have been deployed at the scene.

President Barack Obama walks away from a White House podium after making a statement about the Boston Marathon bombing, 15 April 2013

Speaking at the White House, President Barack Obama said any group responsible would feel the "full weight of justice".