Flags are flown at half-staff traditionally as a symbol of mourning. The president last ordered flags flown at half-staff in December as a memorial gesture to Sen. Daniel K. Inouye (D-Hawaii), the former president pro tempore of the Senate.

On Monday, the president said "the American people will say a prayer for Boston tonight."

"Michelle and I send our deepest thoughts and prayers to the families of the victims in the wake of this senseless loss," he continued.