The guard was identified late Thursday as Lawrence Buckner, 59. He died at Prince George's Hospital Center at 7:19 p.m.
The gunman, described by police Thursday only as a man in his 30s or 40s, left a trail of violence in his wake, NBC Washington reported.
The chaos started when the gunman kidnapped his wife in the 300 block of T Street NE Thursday afternoon, police said. The gunman is also suspected in a shooting on Bruce Place in southeast D.C.
As police were getting word of the kidnapping, the gunman drove to the headquarters of the US Census Bureau in Suitland.
In an exchange of gunfire with police, the gunman was shot multiple times. There was conflicting information about his condition Thursday night.
A D.C. police officer was shot in the leg during the confrontation at 11th and H, and brought into a nearby restaurant for triage. Two other police officers were injured. All the officers have suffered non-life-threatening injuries.
The FBI is investigating the shooting at the Census, which is federal property.