The former sheriff’s deputy accused of failing to respond to this month’s deadly high-school shooting defended himself Monday, saying he believed shots were being fired outside the building where a gunman was actually firing away inside.

The allegations that Scot Peterson “was a coward and that his performance, under the circumstances, failed to meet the standards of police officers are patently untrue,” Joseph DiRuzzo, the attorney for Mr. Peterson, who was assigned to the Parkland, Fla., high school where 17 people were killed earlier this month, said in a statement. The statement said Mr. Peterson properly took up position outside the building.

Broward County Sheriff Scott Israel—under fire himself for how his department handled the shooting response and warnings about the accused gunman—called out Mr. Peterson during a news conference last week for waiting outside and not going into the building. He said Mr. Peterson quit after being suspended without pay.

Speaking last Thursday, Sheriff Israel claimed there was video, which hasn’t been released, showing Mr. Peterson outside the school building for “upwards of four minutes.” He should have “addressed the killer, killed the killer,” the sheriff said.

In response to the two-page statement from Mr. Peterson’s attorney, the sheriff’s office said it cannot discuss details of the case due to an active internal-affairs investigation.