President Barack Obama says the explosions at the Boston Marathon are being investigated as an act of terror, although authorities still don't know who is responsible.
He called the bombing "a heinous and cowardly act" used to target innocent civilians.
Obama spoke to reporters at the White House after a briefing by his national security team.
A 29-year-old restaurant manager has been identified as one of three people killed in the bombing at the Boston Marathon.
Her father says Krystle Campbell, of Medford, Mass., had gone with her best friend to take a picture of the friend's boyfriend crossing the finish line on Monday afternoon.
William Campbell says his daughter, who worked at a restaurant in nearby Arlington, was "very caring, very loving person, and was daddy's little girl." He says the loss has devastated the family.
He says the friend was seriously injured in the explosion.
The president ordered flags at the White House and all government buildings to be flown at half-staff in honor of the Boston Marathon explosion victims.
He 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, including an 8-year-old boy, and more than 170 were wounded in Monday's bombing at the famous marathon's finish line.
According to a person briefed on the probe, the explosives were made of pressure cookers containing metal and ball bearings.