BBC: (Excerpts below)
Multiple sources in the Capitol Police department have told the BBC that its highly trained and heavily armed four-man Containment and Emergency Response Team (Cert) was near the Navy Yard when the initial report of an active shooter came in about 8:20 local time.
The officers, wearing full tactical gear and armed with HK-416 assault weapons, arrived outside Building 197 a few minutes later, an official with knowledge of the incident told the BBC.
'A different outcome'
According to a Capitol Police source, an officer with the Metropolitan Police Department (MPD), Washington DC's main municipal force, told the Capitol Cert officers they were the only police on the site equipped with long guns and requested their help stopping the gunman.
When the Capitol Police team radioed their superiors, they were told by a watch commander to leave the scene, the BBC was told.
The gunman, Aaron Alexis, was reported killed after 09:00.
Several Capitol Police sources who spoke to the BBC asked to remain anonymous for fear of reprisal.
Capitol Police Officer Jim Konczos, who leads the officers' union, said the Cert police train for what are known as active shooter situations and are expert marksmen.
"Odds are it might have had a different outcome," he said of Monday's shooting and the decision to order the Cert unit to stand down. "It probably could have been neutralised."