Aaron Alexis, whom the FBI believes to have been responsible for the shootings at the Washington Navy Yard in Washington, D.C., is shown in this handout photo released by the FBI on Monday. FBI/Reuters/Landov hide caption

itoggle caption FBI/Reuters/Landov

FBI Director James Comey is pictured earlier this month during his swearing-in ceremony at the Justice Department in Washington, D.C. Susan Walsh/AP hide caption

itoggle caption Susan Walsh/AP

Flowers, flags and a child's drawing at a makeshift memorial outside the Washington Navy Yard, where a gunman killed 12 people on Monday. Jonathan Ernst /Reuters/Landov hide caption

itoggle caption Jonathan Ernst /Reuters/Landov

At the White House and around the nation, flags are flying at half-staff since Monday's mass shooting at the Washington Navy Yard. Olivier Douliery /UPI/Landov hide caption

itoggle caption Olivier Douliery /UPI/Landov

Aaron Alexis, whom the FBI believes to have been responsible for the shootings at the Washington Navy Yard in Washington, D.C., is shown in this handout photo released by the FBI on Monday. FBI/Reuters/Landov hide caption

itoggle caption FBI/Reuters/Landov