What the New York Times (Michael S. Schmidt, William K. Rashbaum & Richard A. Oppel Jr.) has on what happened:

[....] The man, Ibragim Todashev, had been speaking for two hours in his apartment to officials from the Massachusetts State Police and the F.B.I. about Mr. Tsarnaev and the Sept. 11, 2011, murders in Waltham, Mass., when he suddenly grabbed an object and tried to attack the agent, one official said.

“He exploded and leapt at him,” said the official, who said the F.B.I. agent sustained minor injuries that required stitches.

A second law enforcement official said the shooting occurred after Mr. Todashev had admitted his role in the killings and had also implicated Mr. Tsarnaev. The official said he had begun writing out a statement when he asked to take a break.

“They got him to confess to the homicides, and they say, ‘Let’s write it down,’ and he starts writing it down. He goes to get a cigarette or something and then he goes off the deep end,” the second official said. “I don’t know what triggered him, and he goes after the agent.”

The official said Mr. Todashev had something in his hand, “a knife or a pipe or something.”

It was not certain who, or how many officers, had fired on Mr. Todashev. Nor was it clear why, with at least three law enforcement officials in the room, deadly force was used on someone without a firearm in his hands. Asked, one law enforcement official said: “If somebody jumps on you and you have a gun, and you don’t do something, the gun will quickly come into play.”

Mr. Todashev’s alleged oral admission and his subsequent death marked a bizarre twist in investigators’ attempts to determine whether Mr. Tsarnaev participated in the gruesome killings in Waltham on the 10th anniversary of the Sept. 11 attacks. The throats of his close friend, Brendan Mess, and two other men were slashed and marijuana was spread over their bodies.

The second official described what in effect appears to be a drug robbery.

“So Tamerlan says they have dope, they rip them off. Tamerlan says, ‘They can identify me, so let’s kill them.’ And they kill them,” the official said. [....]