WASHINGTON (AFP) - A US Census Bureau guard was shot and critically wounded Thursday by a gunman who was reportedly still loose inside the headquarters, US media reported.

The bureau confirmed on Twitter that there had been "an incident at the Suitland Federal Center and there is an ongoing investigation."

The guard was reportedly taken to the hospital in critical condition after sustaining at least one gunshot wound at the bureau's suburban Washington offices located in Suitland, Maryland, CBS affiliate WUSA reported.

Employees inside the facility, which also houses other federal agencies, were not allowed to leave the campus, Census Bureau spokesman Michael Cook said in the Washington Post.