MIAMI — Less than a minute after a sheriff’s deputy screamed into his police radio that shots had been fired at Marjory Stoneman Douglas High School, backup arrived to help — so quickly that the sound of active gunfire could still be heard, according to newly released police accounts of the Feb. 14 mass shooting in Parkland, Fla.

The firsthand police accounts released Friday by the sheriff’s office in Broward County make clear that the gunman was still carrying out his rampage as law enforcement officers started swarming into the school — yet he managed to slip away undetected. The accused gunman, a former student named Nikolas Cruz, was arrested about an hour later, walking down a residential street about two miles away.

A timeline of the police response released last month, citing police radio communications and school surveillance video, indicated that two deputies heard gunfire when they reached the school — one of them, Deputy Michael Kratz, two minutes after the shooting began and the other, Detective Brian Goolsby, three minutes later. The new reports show a third officer, Sgt. Brian Miller, also heard the shots.

Still, it took 11 minutes after the gunman opened fire for police officers from the neighboring city of Coral Springs to enter the freshman building where the shooting took place. By then, he had fled with other students running from the campus.