By PIERRE THOMAS, MIKE LEVINE, JACK DATE AND JACK CLOHERTY

In the moments after Aaron Alexis opened fire inside the Washington Navy Yard on Monday, confusion and conflicting reports reigned. Three days later, on Thursday, FBI Director James Comey sat down with ABC News' Pierre Thomas and several other reporters to offer the most detailed and official account yet of the massacre that killed 12.

Here is Comey in his own words, with minor edits to help readers and organize Comey's comments chronologically:

WHAT HAPPENED THAT MORNING?

"Alexis entered the base with a pass, in a vehicle, at about 8 o'clock on the morning of the shooting. He parked in the parking deck across a small street-way - or almost alleyway - from Building 197." (Before Monday, Alexis had been "working on a server refresh project in" Building 197, and "that would require movement [and access] throughout" the building.)

"[He] entered the building carrying a bag and went up [to] the fourth floor of the building. Entered a bathroom and emerged without the bag and with a Remington 870 shotgun. … A lot of folks have asked, 'Was the shotgun in the bag?' At this point I don't know. I know the shotgun was cut down at both ends - the stock was sawed off and the barrel was sawed off a little bit. And so whether it fit in that particular bag or not, I don't know. Whether it was disassembled or whether he had it hid in some other fashion, I don't know. But he emerged from the bathroom a few minutes after 8 with the shotgun and … almost immediately [started] to shoot at the folks on the fourth floor in a way with no discernible pattern."

"After the shots rang out, people were moving everywhere to try and find safety."

Based on a review of surveillance video, "[Alexis] appears to be moving without particular direction or purpose. That is, his movements don't at least to me appear to have any - as if he was looking for a particular person or particular group. And when you look at the folks who were shot and those poor folks who lost their lives, they're people of all different backgrounds, from all over the building. So I don't discern any pattern."

"It appears to me that he was wandering the halls and hunting people to shoot. And so he shot folks on the fourth floor, the third floor, and then went down to the lobby and shot a security guard and took a weapon from the security guard - a Beretta semi-automatic pistol - and then continued moving up and down through the building, focusing most on the third and fourth floors."

(Comey said he didn't know whether anyone who supervised Alexis was shot, and he didn't know whether Alexis made any comments while executing his rampage.)

Despite earlier reports of Alexis shooting down into an atrium, "I have no evidence that that happened. There's an internal atrium in the building, and he walks around a hallway that opens onto the atrium on the third and fourth floor. But I don't have any reason to believe he shot down into the atrium."

"It appears he had the [shotgun] ammunition in a cargo pocket on the outside of his pants, and ran out of ammunition [at some point], and then began shooting with the gun he had taken off the slain security guard. And that continued until first responders arrived."

"There is a period of time when he is isolated, sort of pinned down, by the responders. But I don't know for sure, sitting here, how long that period is. So the active shooting stops, he's pinned down, and then more responders come. There was an initial group of responders [from the D.C. police, Naval Criminal Investigative Service and U.S. Park Police] who were not a trained active-shooter response team. Then the [tactical] team arrives after he's pinned down."

"And then he was downed and obviously killed at the scene. And so that's the chronology of what happened."

ANY WORD ON WHAT LED TO THIS ATTACK? HAVE AUTHORITIES FOUND A MOTIVE?

"There are indications that this was a person with mental health troubles, and we're just trying to better understand that. … We're trying to make sure we understand in a careful and comprehensive way what happened, and what happened leading up to this."

"We're attempting to understand as best we can his life up until the moment of that shooting, which would include trying to understand whether there were any issues related to work. That's work that's underway now. … We're doing a lot of work to understand his electronic [communications]."

WAS ANYONE ELSE INVOLVED IN PLANNING THE ATTACK?

There is "no sign" that others are connected to Alexis' rampage. "I think early on there was some confusion because a lot of good people ran towards the sound of gunfire with weapons … I don't know this for sure, but it seems reasonable that was the genesis of some of the reporting about there may have been other people with guns. No indication that this was anything but this individual."