Following Monday’s shooting at the Navy Yard in Washington, D.C., President Obama ordered flags lowered to half-staff “as a mark of respect for the victims of the senseless acts of violence.”

Thirteen people, including the shooter, were killed Monday morning when former Navy reservist Aaron Alexis, 34, opened fire at the Washington Navy Yard, authorities said Monday. The incident is the deadliest mass shooting at a military installation in the U.S. Authorities have not released a motive for Monday’s shooting.

Flags are to be lowered through sundown Friday.