I’m going to trust Pete Williams on this, partly because he was the most reliable reporter during the Boston bombings and partly because other outlets like Fox News are confident enough that they’re reporting the same name.

SWAT officers swarmed the building, the headquarters of the Naval Sea Systems Command, and a shooter there was killed, sources told NBC News. Law enforcement officials identified the gunman to NBC News as Aaron Alexis, 34, originally of Fort Worth, Texas. They said he recently began working as a civilian contractor.

Chief Cathy Lanier said at mid-afternoon that as many as two other suspects could be at large, both seen with firearms and wearing military-style uniforms. Minutes later, police said on Twitter that one of the two had been identified and was not a suspect or person of interest…

Terrie Durham, who works at the Naval Sea Systems Command building, said she saw a gunman who appeared to be wearing dark fatigues.

Cops think Alexis used the ID of someone who used to work at the Navy Yard to gain access. How he got that ID is unclear. Maybe that’s where the Rollie Chance reports came from. We’ll see. Per NBC Washington, though, cops increasingly believe that Alexis was the only shooter because the paths taken through the building by the two other suspected gunmen didn’t leave carnage in their wake. One of the other possible shooters was ID’d as a white man in a tan outfit, but he’s now been eliminated as a suspect. I assume he was probably just an MP with his gun out whom someone saw from a distance and couldn’t tell if he was friend or foe.

Another interesting detail from NBC Washington, which is sadly relevant to the new national conversation on guns we’ll soon be having at the Democrats’ and the media’s urging:

Video also shows the gunman entered with a shotgun, News4’s Jackie Bensen reported. He shot the security guard and continued through the building, but according to what witnesses are telling investigators, by the time the shootings ended, the gunman was seen with a semiautomatic 9 mm pistol and an AR-15 assault rifle.

Authorities are investigating whether the gunman took the security guard’s service weapon – likely a 9 mm pistol – and hid in wait for the first responding D.C. police officers, who would be specially armed with AR-15s, then opened fire and shot one of the D.C. police officers in the legs. It’s unclear if the other officers assisting the wounded officer also were able to retrieve his AR-15.

The death toll right now is 11, plus Alexis. People have been e-mailing us a link to a Texas website with a mugshot of a guy named Aaron Alexis from 2010, but after knocking CBS and NBC for identifying the wrong man earlier, I’ll hold off on posting that for now. Look for an update here in case it’s confirmed. As of 4 p.m. ET, there still appears to be no firm clues about his motive.

Update: That was fast. Via Mediaite, NBC Washington says the Aaron Alexis arrested in 2010 — but ultimately not charged — for discharging a weapon is the same guy who went berserk at the Navy Yard.

Update: The FBI is seeking information about Alexis.