President Obama speaks during a memorial service at the Marine Barracks Sunday in Washington, D.C., honoring victims of Monday's shooting at the Navy Yard. Brendan Smialowski/AFP/Getty Images hide caption

itoggle caption Brendan Smialowski/AFP/Getty Images

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

Kathy Gaarde, one of the people killed in Monday's shootings at Washington's Navy Yard, in a family photo. Her husband, Douglass, says the picture depicts Gaarde "with her 94-year-old mother who she cared for until she passed away last year." Douglass Gaarde hide caption

itoggle caption Douglass Gaarde

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

Workers emerge from a building after a deadly shooting at the Washington Navy Yard on Monday. There are multiple injuries and deaths, and one gunman is dead. Police say they are searching for two other "potential" shooters. Alex Wong/Getty Images hide caption

itoggle caption Alex Wong/Getty Images