March 28

TEN MONTHS ago, an FBI special agent shot and killed Ibragim Todashev in the course of interrogating him about a Massachusetts triple murder possibly linked to the late Tamerlan Tsarnaev, the suspected mastermind of the April 15 Boston Marathon bombings. This week, the Justice Department and state authorities in Florida, where the shooting occurred, produced the first detailed official accounts of this bizarre incident. Those long-awaited documents helped settle some key questions while leaving other mysteries unsolved.

Most important, both an internal Justice Department investigation and the parallel probe by Jeffrey L. Ashton, state’s attorney for Orange and Osceola counties, show convincingly that the still-unnamed agent committed no crime or other intentional wrongdoing. He fired on Todashev only after the mixed-martial-arts expert threw a coffee table at the agent’s head, opening a large gash, and then rushed with a metal pole at both the agent and his colleague from the Massachusetts State Police. Investigators relied not only on the testimony of the officers involved but also on independent, corroborating statements from other people and on physical evidence. The fact that Mr. Ashton, an authority outside the FBI chain of command, confirmed the self-defense scenario lends it credibility. The FBI could benefit from independent input in future investigations of lethal force by its agents, as it has historically exonerated them in almost every case.

Alas, it is already too late to prevent the spread of conspiracy theories, especially in Todashev’s native Russia. Fair-minded observers, however, will be reassured that the FBI agent fired in self-defense. Indeed, it would have been strange for him to have acted from any other motive, given that killing Todashev eliminated a key witness and, possibly, a suspect in an unsolved homicide — one who also might shed light on the marathon bombing.

What remains unclear is the precise nature and extent of Todashev’s admissions about the triple homicide in Waltham, Mass., on Sept. 11, 2011 — and what, specifically, he revealed about Tsarnaev’s role. That horrific crime has gone unsolved for too long.

Also, the fact that the FBI agent did not act criminally does not mean that he and his colleagues from Massachusetts law enforcement acted wisely or appropriately at every turn of this highly sensitive investigation. Among the questions the bureau should be exploring is why the agent needed to fire a second volley of three shots after a first one had wounded Todashev. And why was it considered unavoidable to risk a meeting with Todashev in his apartment, where improvised weapons were within his reach, rather than some alternative location?

Yes, it’s easy to second-guess, but the only way to redeem this tragic incident is to learn every lesson from it that can possibly be learned.