On Wednesday, Coral Springs police Sgt. Jeff Heinrich was watering the infield as part of his duties as volunteer baseball trainer at Marjory Stoneman Douglas High in Parkland, Florida, when the school fire alarm sounded and he heard “popping” sounds near the school parking lot.

“I thought it was fireworks and kids messing around at first,” Heinrich tells PEOPLE.

Heinrich was off duty at the time, on campus just as a dad. But then the veteran police sergeant — whose wife is a teacher at the school and son is a student and varsity baseball player there — saw students streaming out of the building and recognized the distinct sound of rapid gunfire.

“I knew I had a shooter on campus,” he says.

The Valentine’s Day school shooting left 17 dead and more than 20 wounded and has since become one of the 10 deadliest mass shootings in modern U.S. history.

Heinrich, who prevented a school shooting at another Florida school in 2016 and was later named Officer of the Year, was among the hundreds of police officers from the Coral Springs Police Department, the Broward County Sheriff’s Office and other police agencies who sprang into action Wednesday.

Coral Springs police Capt. Brad McKeone tells PEOPLE that close to 100 Coral Springs police officers responded to the school shooting and nearly 30 of them went inside the building to help rescue 23 of the injured students.

Meanwhile, Heinrich, who was unarmed, ran towards the parking lot and right into a student who had been hit by gunfire in the left lower leg.

“I got to the parking lot and a kid was coming out of the school screaming he was shot,” he says. “He was bleeding pretty good. It was a pretty nasty gunshot wound.”

Heinrich says he brought the boy to the baseball clubhouse where he grabbed the first-aid kit and attended to his wound.

He then called police dispatch, with the student aiding in providing a description of the alleged shooter, who was later identified as 19-year-old former student Nikolas Cruz.

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After he called police, Heinrich says he phoned his wife and son who were still on campus, hiding in the same classroom in the culinary building near where the gunman, armed with an AR-15 assault rifle, had opened fire on students as they exited their classrooms following the fire alarm he had allegedly triggered.

“I told them this is real and stay locked in the classroom,” Heinrich says.

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Once paramedics arrived and tended to the wounded student, Heinrich ran to meet up with the swarm of officers already on the scene. He grabbed a colleague’s spare gun, put on a police vest and helped patrol the area with other officers looking for the shooter and helping the students.

“There is nothing to prepare yourself for this but you have to resort back to your training and it kicks in and overpowers your first thought process,” he says. “It is the most horrific event you could possibly deal with as a human being.”