At least three people are dead, including an eight-year-old, and roughly 140 injured after two bombs exploded near the finish line of the Boston Marathon on Monday.

The bombs went off about 15 seconds apart around 3 p.m. Monday. The explosions occurred long after the winner of the marathon crossed the finish line, but the number of casualties is expected to rise due to the large crowds gathered near the end of the race to watch remaining runners come in.

The FBI is investigating the bombings which it has classified as a terrorist attack.

U.S. President Barack Obama pledged full support toward the investigation, saying whoever is responsible "will feel the full weight of justice."

Some 23-thousand runners took part in the storied, 116-year-old race.