FORT LAUDERDALE, Fla. (AP) — The police response to the Florida high school massacre was delayed because school officials rewound a school surveillance video, making officers think the gunman was still in the building, a new report shows.

The South Florida Sun-Sentinel reports documents show then-Marjory Stoneman Douglas assistant principal Jeff Morford and security officer Kelvin Greenleaf began viewing a livestream from inside the three-story freshman building minutes after the Feb. 14 shooting stopped.

When they didn't see the gunman who killed 14 students and three staff members, they rewound the video until they found him and began relaying information to Broward Sheriff's Deputy Scot Peterson, who radioed officers who had charged into the building.

At one point, officers were told suspect Nikolas Cruz was coming down the stairs when he had left the building nearly 30 minutes earlier, abandoning his AR-15 semi-automatic rifle. Officers told investigators that delayed their arrival on the third floor, where several students were dead and wounded. One deputy said one girl had a pulse when they found her, but she was dead by the time they got her outside.

"Had we known the shooter wasn't there, we probably could have flooded that building a lot faster knowing that we're just going to go in there and just start trying to recover victims and wounded people," Coral Springs police SWAT officer George Schmidt told investigators.

The video delay came to light months ago, but it was not clear then how the misunderstanding occurred.

According to reports, Morford heard the fire alarm — it was probably activated by smoke from Cruz's gun — and raced outside, where he found Peterson, who yelled, "Code Red," the school district's term to lockdown the school because of a shooter. Peterson told Morford to go to the school's security room and look at the livestream from surveillance cameras inside the freshman building.

Morford found Greenleaf already there, but they couldn't locate the shooter. They rewound the video until they found Cruz and Morford radioed Peterson what they saw — but he doesn't remember if he told Peterson the information was nearly a half-hour old. Peterson told officers inside the building that Cruz was coming down the stairwell from the third floor, so they waited to ambush him — not know that he had already fled the school.

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That delay cost precious minutes.

"They brought a young lady down from the third floor. I guess she had a pulse up top. When they got back downstairs . she was gone," sheriff's Sgt. Christopher Palmara told investigators.

Peterson retired shortly after it was revealed he failed to enter the building to confront Cruz. Morford was recently transferred to another job while the school district investigates his actions before the shooting, including whether he brushed aside students' concerns about Cruz, who attended the school in 2016 and 2017. He denies having those conversations, but the state commission investigating the shooting concluded he was not being truthful.

Cruz, now 20, has pleaded not guilty but his attorneys have said he would plead guilty in exchange for a life sentence. Prosecutors are seeking the death penalty.