Probe launched over claim that elite Capitol Police unit blocked from Navy Yard massacre

The board that oversees the U.S. Capitol Police has opened an investigation into whether a tactical team of officers that was one of the first on the scene during the Washington Navy Yard shooting was ordered to stand down.

Several sources confirmed the probe to Fox News. The investigation follows reports that a highly trained and specialized Capitol Police team arrived soon after the shooting started, but was told by a supervisor to leave the scene.

The BBC, which first reported on the allegation, quoted a Capitol Police "source" as claiming "lives may have been saved" if the team could have intervened.

Aaron Alexis killed 12 people at the Washington Navy yard before he, too, was killed. The emerging allegations raise questions as to whether the tactical team could have stepped in earlier to stop the violence.

U.S. Capitol Police Chief Kim Dine said the force "offered and provided mutual support and assistance" on Monday, and that there is a "preliminary investigation into the allegations presented."

House Administration Committee Chairwoman Candice Miller, R-Mich., also said she's been advised "there is currently an active investigation into the allegation."

According to the BBC, the Metropolitan Police Department -- the police force for Washington, D.C. -- is denying the claims. One MPD officer was also wounded in the shooting.

But an official with the Capitol Police union said the Containment and Emergency Response Team (Cert) -- the tactical team in question -- trains for active shooter situations.

They reportedly arrived near the scene within minutes of the report of an active shooter going out and were armed with HK-416 assault weapons.

The Capitol Police Board, which is investigating, consists of House Sergeant at Arms Paul Irving, Senate Sergeant at Arms Terry Gainer, Architect of the Capitol Stephen Ayers and Dine. The House and Senate assumed radically different security postures later in the day Monday, with Gainer ordering the Senate on a lockdown in the late afternoon, while the House remained open.

The board has set up a "fact review team" to investigate the Capitol Police response. The results are due no later than Oct. 21.

Fox News' Chad Pergram contributed to this report.