Update, 4:53 p.m.: The Washington Post reports that the guard was 31 or 32 years old, and he shot himself with his department-issued revolver.
The Smithsonian is tweeting about a possible suicide that has taken place inside a non-public area inside the Hirshhorn Museum.
Shortly after 4 p.m., the Smithsonian account tweeted, “Police are investigating death of @hirshhorn security guard by self-inflicted gunshot wound. The museum has been evacuated and closed.” No one else was injured.
Smithsonian blog Around the Mall says law enforcement is on the scene. Architect magazine reports that the suicide took place in a locker room inside the museum. Twitter user @ebuie posts this photo from the scene outside the Hirshhorn, where emergency response vehicles clustered after the victim was found.