Mr. Peterson should have “went in, addressed the killer, killed the killer,” Sheriff Israel said.

“Let there be no mistake, Mr. Peterson wishes that he could have prevented the untimely passing of the seventeen victims on that day, and his heart goes out to the families of the victims in their time of need,” Mr. DiRuzzo said in a statement. “However, the allegations that Mr. Peterson was a coward and that his performance, under the circumstances, failed to meet the standards of police officers are patently untrue.”

It is unclear why Mr. Peterson nevertheless chose to retire on Thursday after Sheriff Israel placed him under internal investigation. Sheriff Israel has said the investigation will continue despite the deputy’s departure. The sheriff’s office is under intense scrutiny over its response to the Feb. 14 shooting.

The office’s active shooter protocol says that on-scene deputies, without approval from a supervisor, “may” engage the gunman. That appears to give deputies discretion, but only to a point: They “may” choose not to go in if they know a door is armed with a large explosive device, for example, or if they know the only way into a building has a dangerous suspect immediately on the other side ready to shoot the deputy, Col. Jim Polan explained in written responses to questions.

Otherwise, deputies are expected to prioritize the lives of victims by trying to confront the gunman as quickly as possible.

Sheriff Israel said on the day of the massacre that it appeared the shooting had begun and then continued outside the building, and that three people had died outside the school. But on Monday, his office said, “The shooting occurred inside.”