Two bombs ripped through the crowd at the finish line of the Boston Marathon on Monday (April 15), killing three people, causing several amputations and injuring more than 100 in what a White House official said would be handled as an "act of terror."

It was the worst bombing on U.S. soil since security was tightened after the attacks of September 11, 2001, and President Barack Obama promised to hunt down whoever was responsible for the attack on a day when tens of thousands of spectators packed the streets to watch the world-famous race.

Many runners were heading for the finish when a fireball and smoke rose from behind cheering spectators and a row of flags representing the countries of participants, video from the scene showed.

Ambulances, fire trucks and dozens of police vehicles converged at the scene, and spectators could be seen crying and consoling each other.

The blasts put police on alert in major cities across the United States, including in Washington, D.C. and New York City, sites of the September 11 attacks.

Presented by Adam Justice