The Florida school shooter was able to walk around in public after his massacre — completely undetected — for nearly a half-hour thanks to a mix-up with the security cameras, which caused cops to think he was still on campus, when in fact he was already gone.

“He went from the third floor to the second floor … They’re monitoring him on camera,” an officer can be heard saying over radio transmissions, which were reviewed by the Sun Sentinel.

“We’re on the second floor, we don’t see him,” another cop said.

Authorities spent 26 minutes searching for alleged gunman Nikolas Cruz before eventually realizing that he wasn’t inside Marjory Stoneman Douglas High School anymore, according to the Sentinel.

The 19-year-old managed to slip out unnoticed after his 10-minute shooting spree due to a delay on the school’s surveillance tapes.

Deputies from the Broward County Sheriff’s Office reportedly arrived at the Parkland, Fla., campus at the same time Cruz was escaping. While it’s unclear from radio transmissions if cops responded to the right building at first, the tapes confirm the malfunction with the security footage.

“It’s about a 20-minute delay they’re following him on video, on the camera,” an officer can be heard saying over the radio, after realizing their mistake.

“They have him exiting the building, running south.”

Broward Sheriff Scott Israel said at a press conference Wednesday that investigators were “examining” the tapes and other evidence to see if deputies immediately entered the school upon their arrival.

Cruz was caught on camera dropping his rifle and fleeing Stoneman Douglas around 2:28 p.m. last Wednesday. Cops didn’t catch him until more than an hour later.

“I’ve never heard of that problem before,” Pete Blair, a criminal justice professor at Texas State University, said of the camera delay.

“That’s going to slow you down because you think that’s good information, but it’s not good information.”

Coral Springs Police Chief Tony Pustizzi confirmed that the surveillance tapes offered to police by the school were “delayed 20 minutes and nobody told us that.”

“It made it harder to identify where the guy was,” he told the Sentinel. “Somebody would say, ‘He’s on the second floor,’ and we had guys on the second floor saying, ‘We’re on the second floor, we don’t see him.’ That’s when we figured out there’s a tape delay.”

Pustizzi added, “Once we found that out, we were able to adjust.”