Media playback is unsupported on your device Media caption CCTV footage shows Aaron Alexis stalking the corridors inside the Navy Yard complex

The union representing officers with the US Capitol Police (USCP) has called for a congressional inquiry into the decision to pull a Swat team from the scene of a mass shooting in September.

A USCP tactical response team arrived first at the Washington Navy Yard but was told by a supervisor to leave instead of entering the building.

Aaron Alexis, a 34-year-old former US Navy reservist, killed 12 people.

The USCP union called a department investigation into the matter "flawed".

'Biased'

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.

The Capitol Police Labor Committee, which represents hundreds of officers, has called for closed hearings conducted by the House of Representatives administration committee, "in an effort to determine the truth".

In a statement obtained by the BBC, the committee criticised a recent internal inquiry as inadequate and "biased and designed to protect the department from further scrutiny and possible litigation from the victims' families".

Investigators empanelled by the US Capitol Police Board - known as the Fact Review Team - released their findings on the Friday afternoon before a US holiday weekend.

The panel acknowledged a four-person team of heavily armed officers, known as the Containment and Emergency Response Team (Cert), did in fact deploy on its own initiative to the scene of the shooting, but were "instructed not to enter the Navy Yard".

The panel put the Cert team's inability to reach a Navy Yard incident command post down to "traffic gridlock", and said the team was later withdrawn due to "uncertainty about threats to the US Capitol and absent a request for additional USCP assets".

The Fact Review Team was led by Michael Stenger, a former assistant director of the US Secret Service.

'Running battle'

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

On 16 September, Alexis entered Building 197 of the Washington Navy Yard about 8:15 local time (12:15 GMT) and began shooting unarmed civilian Navy employees.

The BBC has revealed that the force's highly trained and heavily armed four-man Cert team were near the Navy Yard when the initial report of the shooting 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 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.

But when the Capitol Police Cert team radioed their superiors, they were told by a watch commander to stand down.

An FBI investigation later determined another set of armed officers did not arrive to engage Alexis for nearly 15 more minutes, at which point all 12 victims are believed to have already been killed.

Alexis was shot dead by police.

'Damage control'

Capitol Police Labor Committee Chairman Jim Konczos said the department was conducting "damage control". He said the US Capitol, the seat of the US House of Representatives and Senate, was adequately secured by other heavily armed police.

"Allowing four highly trained officers to assist other law enforcement agencies at the Navy Yard would not have impacted the security of the Capitol," the police union said in the statement obtained by the BBC.

In an interview, Mr Konczos said the department's officers were "disgusted" with the handling of the matter and said they had "no reason to lie" about the events surrounding the mass shooting.

The union leader also called into question the Fact Review Team's independence, saying it could not conduct an unbiased investigation of a department it oversaw.

Senate Sergeant-at-Arms Terry Gainer, who has authority over the USCP, told the BBC the Fact Review Team was well qualified to conduct an independent investigation.

"I believe the review was thorough," he said.

Acknowledging the department was "less crisp on handling spontaneous events", Mr Gainer said the department was weighing improvements to internal communications in the event of such security threats.

But he disagreed with the Labor Committee's assessment of the Navy Yard shooting response.

Victims 'deserve better'

Meanwhile, the Labor Committee has called on the House administration committee - which oversees matters related to the USCP - to investigate further.

The victim's families "deserve better" than the findings of the Fact Review Team, Mr Konczos said.

The family of shooting victim Mary Knight has filed a $37.5m (£23.4m) administrative claim against the US Navy and the Department of Veterans Affairs, claiming they ignored red flags about Alexis's deteriorating mental health leading up to the shooting.

The former Navy contractor believed he was being controlled by electromagnetic waves, had a history of violent outbursts and paranoia, and had been arrested multiple times in recent years, according to the FBI.

Knight family lawyer Sidney Matthew said he was looking closely at the USCP Navy Yard findings and any subsequent investigations into the matter.

"If the Capitol Police were there and mobilised and turned loose, if they were not told to stand down, the question becomes would they have taken Alexis out before he shot more of the victims," he said. "The potential was there... Now the question is, did it make a difference. That's what we need to know."