WASHINGTON, D.C. -

President Barack Obama has ordered flags to be flown at half-staff at the White House and all public buildings and grounds and military posts and stations throughout the United States as a mark of respect for the victims of the Boston Marathon explosions.

Three people were killed and at least 170 injured after two bombs went off near the finish line of the marathon Monday afternoon.

President Obama called it "a heinous and cowardly act." During a news briefing Tuesday morning, the President said the FBI is investigating the bombings as an act of terrorism but they did not know who carried out the attack.

During the news conference, the president praised emergency crews and people who risked their lives to save others. President Obama said some Boston Marathoners kept running to the hospital to give blood, while others ran to the scene of the bombings to help others.

The flags will be flown at half-staff until sunset on April 20.