US President Barack Obama speaks on the Boston Marathon bombings on April 16, 2013 in the Brady Briefing Room of the White House (Getty Images)
WASHINGTON -- President Obama declared the Boston Marathon bombing an "act of terror" on Tuesday, but said investigators don't know who did it or why.
"This was a heinous and cowardly act," Obama said at the White House.
Obama spoke shortly after ordering flags at the White House and other government buildings be flown at half-staff in honor of the victims of Monday's attack.
FBI Director Robert Mueller, counter-terrorism adviser Lisa Monaco, and other officials briefed Obama on Tuesday morning.
The flag lowering is "a mark of respect for the victims of the senseless acts of violence perpetrated on April 15, 2013," Obama said in a proclamation.
The bombing has killed at least three people and injured more than 170 others.
In remarks Monday evening, Obama said officials still don't know who carried out the bombing, or why.
"People shouldn't jump to conclusions before we have all the facts," Obama said. "But make no mistake -- we will get to the bottom of this ... Any responsible individuals, any responsible groups will feel the full weight of justice."