FOR REASONS that are unknown, in circumstances that remain murky, a young man was shot and killed in Alexandria the other night by an off-duty Arlington County sheriff’s deputy. Based on the results of an autopsy, the shooting of Julian Dawkins, a 22-year-old shuttle driver for PBS’s “NewsHour” program, was ruled a homicide. But the deputy, Craig Patterson, was neither arrested nor charged. Why?

Police and prosecutors say they are loath to release information in an ongoing investigation; they are reluctant to prejudice the outcome of the case by discussing it in the media.

But the skeletal facts that are known about Mr. Dawkins’s death early Wednesday demand an explanation. Residents of Alexandria, where there has been just one other homicide in the last year and a half, are within their rights to expect timely answers.

According to a person familiar with the investigation, Mr. Patterson said he acted in self-defense, shooting Mr. Dawkins once in the chest, after Mr. Dawkins threatened him with a knife. Police did recover a knife on the scene from Mr. Dawkins, this person said. Mr. Patterson called police after the shooting and remained on the scene to be questioned once officers arrived.

However, a woman interviewed by The Post’s Matt Zapotosky and by police has cast doubt on Mr. Patterson’s version of events. She said that she heard Mr. Dawkins (whom she knew) arguing with another man, apparently Mr. Patterson. She said she saw Mr. Dawkins briefly chase the man down the street and heard the man yell back, “You best believe I’ll be back.” And she said that after some time had elapsed — perhaps 5 to 15 minutes — she heard a gunshot and looked outside to see a man standing in a neighbor’s yard near the face-down body of Mr. Dawkins.

If that witness’s account is correct — and we don’t know whether it has been corroborated by other witnesses or evidence — it raises troubling questions about Mr. Patterson’s actions. Specifically: Why didn’t he call the police instead of returning to the scene to confront Mr. Dawkins?

Another potential witness, Kim Bragg, who is Mr. Dawkins’s aunt, told The Post she was on the phone with him at the moment he was shot. What kind of threat could Mr. Dawkins have posed at the time of his death to Mr. Patterson if he was speaking on a cellphone?

Mr. Patterson, 44 years old, a 17-year veteran of the Arlington sheriff’s department, has been put on paid administrative leave pending an investigation. The authorities in Alexandria say he is being treated as any other suspect would be in a major crime investigation and will be accorded no special preference by dint of his badge.

Mr. Patterson is innocent until proven guilty, and no one should rush to judgment against him. By the same token, police and prosecutors in Alexandria owe city residents a full and detailed accounting of the events that led to Mr. Dawkins’s death.