Mr. Peterson nevertheless remained outside, and he told other officers arriving on campus to do to the same. “Do not approach the 12 or 1300 building,” Mr. Peterson said. “Stay at least 500 feet away.”

Mr. Peterson resigned eight days after the shooting once the sheriff, Scott Israel, placed him under internal investigation for failing to try to confront Mr. Cruz as quickly as possible, as outlined in office policy. Mr. Peterson’s lawyer defended his client a few days after his resignation, saying Mr. Peterson thought the gunman was outside the building — something that appears at odds with Thursday’s timeline.

The 911 calls came into the sheriff’s office through a regional county system. The first police officers to respond to the shooting came from the city of Coral Springs, which is near the school and operates its own, separate call system.

Using the adults as intermediaries, the recordings revealed, county dispatchers tried to gather information and make recommendations about how the students could shelter inside the school.

“Tell her to be as safe as she can,” a dispatcher told a woman whose granddaughter was in the school. “If she can find somewhere secure, then do so, but use her best judgment.”

Only one of the recordings was of a call from inside the school — the one from the unidentified boy, who was speaking too softly for the dispatcher to hear him clearly.

“I’m sorry, I can’t hear you,” the operator responded. “What’s happening?”

“Somebody is shooting up Stoneman Douglas,” he said.