PHOENIX — Dozens of Phoenix police officers rushed to a dark vacant lot near the State Capitol here last October, responding to a radio call of an officer down. What they found continues to haunt them a year later: Sgt. Sean Drenth, a well-liked 12-year veteran, sprawled on his back, dead from a blast from his own police-issue shotgun.

Twelve months after the shooting, the investigation remains wide open, with the police still not sure whether Sergeant Drenth, 34, took his own life or it was taken from him. Because there is evidence to support both suicide and homicide, the case has been classified as “death unknown.”

Within any police department, there is understandable outrage, along with calls for justice, when an officer is killed. When an officer commits suicide, there is soul-searching and sadness. In this case, though, there are just questions.

“People want closure,” said Will Buividas, treasurer of the Phoenix Law Enforcement Association, the union representing officers. “They want progress. But we’re in a holding pattern. You have people who are adamant he could never commit suicide, and you have people on the other side. A year later, the case is still hanging out there.”