The sheriff’s deputy who was assigned to the Florida school where a gunman killed 17 people last week has left the force after evidence showed he stood outside the building where the shooting occurred rather than going in, Sheriff Scott Israel said Thursday.

“He never went in,” Sheriff Israel said, describing video footage of the scene he had watched.

Asked what school resource officer Scot Peterson, who was the only armed security guard at the school, should have done, the sheriff said he should have gone into the building, “addressed the killer, killed the killer.”

This is the latest in a series of lost opportunities to stop Nikolas Cruz, the accused gunman at Marjory Stoneman Douglas High School in Parkland, Fla., either before or during the attack. The Federal Bureau of Investigation last week said a specific tip identifying Mr. Cruz’s threat in January fell through the cracks.

Two unnamed Broward County deputies also have been placed on restricted duty while the department explores what could have, or should have, been done when police responded to calls regarding Mr. Cruz before the shooting, Sheriff Israel said.

The sheriff said Mr. Peterson arrived about a minute and a half into the six-minute shooting and stood outside for “upwards of four minutes.”

“What matters is that when we in law enforcement arrive at an active shooter, we go in and address the target. And that’s what should have been done,” the sheriff said. The video hasn’t been released publicly.

When asked about how he felt about the issue, “devastated, sick to my stomach,” Sheriff Israel said. “There are no words.”

Mr. Peterson resigned after Sheriff Israel suspended him without pay pending an internal investigation, the sheriff said, speaking at a news conference.

Mr. Peterson couldn’t be reached for comment Thursday.

Jeff Bell, president of the Broward Sheriff’s Office Deputies Association, said Mr. Peterson is a veteran. He said deputies are trained “to go in and neutralize the threat” when dealing with active shooters.

“That apparently did not happen,” said Mr. Bell, who said he spoke with Mr. Peterson Thursday morning.

New details emerged Thursday about confusion some law-enforcement officers from the nearby town of Coral Springs experienced shortly after the shooting. Some police officers who thought they were monitoring the shooter’s real-time movements inside the school on surveillance cameras were actually seeing video that had been recorded 20 minutes earlier, officials said.

“When the person was reviewing the tape from 20 minutes earlier, somehow that wasn’t communicated to the officers that it was a 20-minute delay,” Coral Springs Police Chief Tony Pustizzi said at a news conference.

According to a timeline provided by the Broward County Sheriff’s Office, the shooter was in the school for about seven minutes, then ran toward tennis courts and joined others who were fleeing. Mr. Cruz was later arrested without incident in Coral Springs.

The confusion over the video recording “never put us in a situation where any kids’ lives were in danger or any teachers’ lives were in danger,” the chief said.

Broward County school officials said the surveillance system allows users to watch both live and recorded video. “During the immediate response to the event, the system was being viewed in real-time and the recorded footage was being viewed to retrace the actions of the shooter,” school officials said in a statement.

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