WASHINGTON (AP Apr 16, 2013) — President Barack Obama said authorities still are trying to find out who set off the bombs Monday at the Boston Marathon that killed three people and injured more than 170 others.

Speaking from the White House at 11:30 a.m.Tuesday, Obama said officials remained unsure whether an individual, more than one person or an organization planted the devices and had “no sense of a motive yet.”

He promised to keep the public posted as information developed.

Earlier Tuesday, Obama ordered flags at the White House and all government buildings to be flown at half-staff in honor of the Boston Marathon explosion victims.

Obama signed a proclamation making the order Tuesday morning, calling it a “mark of respect for the victims of the senseless acts of violence” perpetrated in Boston the previous day. He ordered flags to remain lowered through sunset Saturday.

Three people were killed, and more than 170 were wounded in Monday’s bombing at the famous marathon’s finish line.