A tactical unit of heavily-armed Capitol Police officers was near the scene of the Washington Navy Yard shooting as it happened, ready and capable of stepping in. But according to multiple sources speaking to the BBC, that unit was ordered to stand down and leave the scene, instead of helping the Municipal Police Department take down the shooter.

The officers, part of a four-man Containment and Emergency Response Team responsible for guarding the nearby Capitol, were "wearing full tactical gear and armed with HK-416 assault weapons" when they arrived near the scene of the then active mass shooting on Monday morning, according to the BBC. Here's what allegedly happened next: 

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 assistance in searching for the gunman.

When the Capitol Police team radioed in to their superiors, they were told by a watch commander to leave the scene, the BBC was told.

The Capitol Police are pulling radio logs from Monday morning to investigate the incident, according to a spokesperson for the agency, who added that the department provided "mutual support and assistance" in the aftermath of the shooting. Senate Sergeant-at-Arms Terry Gainer told the BBC that "It's a very serious allegation and inference to indicate that we were on scene and could have helped and were told to leave...it crushes me if that's the case." Gainer's position includes oversight of the Capitol Police. Meanwhile, a spokesperson for the MPD told the BBC that the allegations of an initial order to stand down were "not true." Maybe this was a misunderstanding during the moment or another one after the fact, but this could certainly loom large as the investigation unfolds.

The flow of new developments from Monday's tragedy has slowed to a trickle as the investigation continues. Here's what else we learned on Wednesday:

—Investigators have revealed two strange phrases gunman Aaron Alexis carved into his gun before the shootings: “Better off this way” and “My ELF weapon.” Noting that officials don't know what the phrases were intended to mean, the Washington Post explains that ELF is generally understood to stand for “extremely low frequency," though it could have another meaning in this context. 

—The U.S. also heard from Alexis's mother on Wednesday morning. In a heartbreaking statement read in Brooklyn by a local pastor, Cathleen Alexis said: 

"Our son Aaron Alexis has murdered twelve people and wounded several others. His actions have had a profound and everlasting effect on the family of these victims...I don’t know why he did what he did and I’ll never be able to ask him why. Aaron is in a place where he can no longer do harm to anyone. For that I am glad. To the families of the victims, I am so very sorry that this happened. My heart is broken.”

Click here for our full coverage of the Navy Yard shooting: