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In the minutes after a gunman opened fire in a Florida high school, killing 17, frantic students and parents begin flooding 911 with calls.

A deputy on the Marjory Stoneman Douglas High School campus first thought the loud bangs were firecrackers but quickly realized they are gunshots — yet he never ran toward them.

Other responding deputies and police officers desperately tried to sort through a chaotic scene, treat the injured, lock down the school and locate the shooter.

The Broward County Sheriff’s Office on Thursday released 12 minutes of radio transmissions from its deputies and neighboring Coral Springs police, along with recordings of 10 of the 81 calls its 911 center received during the Feb. 14 shooting. The sheriff also released a written timeline laying out how the radio calls correlated with what was seen on unreleased school security video.

Investigators say video shows suspect Nikolas Cruz opening fire with an AR-15 assault rifle 15 seconds after he enters the school’s freshman building, and firing periodically over the next six minutes. Deputy Scot Peterson, the resource officer assigned to protect the school, is at the nearby administration building. It will be more than 90 seconds before he heads toward the shooting. The first 911 call comes in 68 seconds after Cruz opens fire. The first responding deputies arrive two minutes after that.

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The 911 calls came from students hiding in the freshman building’s classrooms and parents who were getting calls and text messages from their children.

The first calls show the operators’ confusion. A male inside the school, possibly a student, whispers, “There’s shots at Stoneman Douglas, Someone is shooting up the school at Stoneman Douglas.”

“I’m sorry. I can’t hear you. What’s happening?” the dispatcher responds.

“Someone is shooting up?Marjory Stoneman Douglas” he whispers. She still can’t hear him, “Hello… hello…hello.”

But soon a sense of order begins to emerge among the Broward dispatchers and they start giving instructions on keeping the students safe. Just 13 months earlier, the same 911 center had handled a mass shooting at Fort Lauderdale’s airport that left six dead.

In a call relayed from nearby Boca Raton, lasting more than 16 minutes, a man reports information from a mother standing beside him. She’s on another line with her daughter who is in a classroom with just one other girl. They have no closet or enclosed desks to hide behind.

The 911 dispatcher soon instructs the man to tell the girls to remain silent and turn off their cellphone ringers in case the shooter is nearby.

As the call drags on, the mother can be heard in the background encouraging the girl, who hears noises in the hallway. “I love you, I love you. It’s going to be fine if you hide somewhere. Can you play dead? You need to fake dead,” the mother tells the girl.

Seconds later, officers burst into the room and the girls are safe. The mother can be heard telling her daughter, “Tell them to pray, tell them to pray for strength.” The two girls are led out and the call ends.

The operator sighs, “‘Oh my God.”

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Outside the freshman building, Peterson makes his first radio call almost two minutes after Cruz first fired. “Be advised we have possible, could be firecrackers, I think we have shots fired, possible shots fired,” he tells dispatchers. Investigators say 18 seconds later he took up a position near the building and remained there for several minutes. His subsequent transmissions focus on getting nearby streets and the school shutdown and keeping deputies away from the building. Deputies set up a perimeter. Sheriff Scott Israel has said Peterson should have charged into the building and killed Cruz. Peterson, who has denied wrongdoing, retired rather than accept a suspension and investigation.

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By now, students are flooding out of the school. Officers from nearby Coral Springs are arriving to assist deputies. Soon, their calls appear to be more aggressively assessing what they face.

About this time Cruz discards his jammed gun. His burgundy hoodie from his days as member of the school’s Junior Reserve Officer Training Corps allows him to mix into the fleeing mass and get away.

Deputies and officers begin finding their first victims outside the building. A student shot in the leg. Another wounded by an entrance, another in the parking lot. A staff member not moving.

Eleven minutes after Cruz first fired — five minutes after he fled — four Coral Springs officers and two deputies enter the freshman building. More deputies and officers soon follow. They find bodies. They find frightened teenagers huddling in classrooms.

Another mother on a 911 call hears her daughter’s rescue but then exclaims, “Three shot in her room. Oh my God. Oh my God.”

An hour and 19 minutes after the first shots were fired, an hour and 13 minutes after Cruz left the building, 47 minutes after Cruz bought a soda at Walmart, Coconut Creek police officer Michael Leonard turns onto a quiet suburban street about a mile south of the school. He spots a teen wearing a burgundy hoodie walking. He yells at Cruz to get on the ground.

He does.

(AP)