WASHINGTON - US President Barack Obama will head to Boston on Thursday after the deadly explosions rocked the city's well-known marathon race, the White House announced on Tuesday in a statement.

"On Thursday morning, the President will travel to Boston to speak at an interfaith service dedicated to those who were gravely wounded or killed in Monday's bombing near the finish line of the Boston Marathon," the statement said.

Earlier on Tuesday, the president made his second televised statement following the explosions which rocked the Boston Marathon finish line, killing three and injuring over 170 people.

Obama said the FBI was investigating Monday's explosions as "an act of terrorism."

He said the authorities still did not know "who carried out this attack or why, whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual." But he vowed again that the government will "find whoever harmed our citizens" and "will bring them to justice."

The twin bombs exploded at around 3 pm local time (1900 GMT) Monday within seconds of each other near the finish line of one of the world's most revered races.