Image caption Personnel returned to work at the Washington Navy Yard on Thursday

Members of a Washington DC Swat team who the BBC has learned were ordered not to respond to Monday's Navy Yard shootings have yet to be contacted by the authorities.

The Capitol Police tactical response team was told by a supervisor to leave the scene instead of aiding municipal officers, sources told the BBC.

Meanwhile, the department has installed a new leader of the elite unit. No reason has been given for the decision.

Gunman Aaron Alexis killed 12 people.

The BBC has also learned that four members of the highly trained team have applied for temporary leave, as they "grapple" with the aftermath of the incident.

The Capitol Police department has not yet granted the request, nor given approval for them to use their own paid time off.

Command shake-up

Days after the shooting, none of the officers has been questioned by officials or investigators from a special panel that was convened on Wednesday.

Navy Yard shooting timeline 8:00 (12:00 GMT): Aaron Alexis parks car in a multi-storey car park opposite Navy Yard and enters Building 197 using security pass. He moves to a fourth-floor bathroom and removes a shotgun from a bag.

Aaron Alexis parks car in a multi-storey car park opposite Navy Yard and enters Building 197 using security pass. He moves to a fourth-floor bathroom and removes a shotgun from a bag. 8:15: Alexis leaves bathroom and begins shooting people at random in hallways and offices.

Alexis leaves bathroom and begins shooting people at random in hallways and offices. 8:20: Reports surface of active shooter at Navy Yard.

Reports surface of active shooter at Navy Yard. 8:36: A Capitol Police Cert team arrives at Navy Yard and is asked by a Metropolitan Police officer to join the search for the gunman. They are instructed by a supervisor to leave the scene.

A Capitol Police Cert team arrives at Navy Yard and is asked by a Metropolitan Police officer to join the search for the gunman. They are instructed by a supervisor to leave the scene. 8:50: Armed police arrive and engage Alexis, according to FBI Director James Comey. All 12 victims are believed to be already dead or dying. Running-battle with police ends in Alexis' death.

Members of the Containment and Emergency Response Team (Cert) are typically debriefed "right away, at the very least the following day" after an incident, a Capitol Police source said.

"[They] haven't even been given the courtesy of a debrief… They have not even been given an answer as to why the decision was made that they should not respond," the source added.

Another Capitol Police source close to the incident told the BBC: "No-one's talked to these officers since this happened."

On Thursday, members of the tactical unit, which has several dozen members, were told that it had a new leader, a Capitol Police source said. It is not clear why the command shake-up occurred.

Four Cert team members wearing full tactical gear and armed with HK-416 assault weapons arrived on scene at Navy Yard at 08:36 (12:36 GMT) on Monday, after reports surfaced of an active gunman at the complex at 08:20.

According to sources, 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 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.

On Thursday, FBI Director James B Comey Jr told ABC News it took roughly half an hour for armed police to arrive and engage Alexis. All 12 victims were killed within that time.

Capitol Police sources suggest "lives may have been saved" if the Cert team had been allowed to intervene.

'Better off this way'

On Wednesday, Capitol Police Chief Kim Dine appointed a "Fact Review Team" to investigate the department's response to the Navy Yard shootings. It will report back on 21 October.

Aaron Alexis Defence subcontractor IT worker who had a valid pass for Navy Yard

Reportedly suffered from sleeplessness, paranoia and hearing voices

Full-time US Navy reservist from 2007-11, ranked petty officer 3rd class

Said to have been Buddhist convert, he had gun-related brushes with law in 2004 and 2010 Profile: Aaron Alexis

But on Thursday sources questioned the efficacy of such a panel.

They said Capitol Police officers had more confidence in the results of an independent FBI investigation which aims to uncover the facts surrounding the incident.

One officer close to the situation called the lack of communication from department leadership "unforgivable".

Details of the gunman and the shooting continue to emerge.

Alexis, a former Navy reservist, reportedly had a history of mental health problems and gun-related brushes with the law.

He had sought treatment for insomnia at two Veterans Affairs hospitals in the last month, according to media reports.

Law enforcement officials said the IT subcontractor had carved the messages "better off this way" and "my ELF weapon" on the stock of his sawn-off Remington 870 pump-action shotgun.

All victims were killed in hallways or their offices, not in the building's atrium as media outlets previously reported.

Alexis also gunned down a security guard and took his Beretta handgun, Mr Comey said.