A hard-to-find key to a locked classroom door was the ultimate reason police waited 77 minutes to enter a Robb Elementary classroom to kill a gunman, stopping the massacre that claimed the lives of 19 students and two teachers, the under-fire police chief said.

Uvalde school district police Chief Pete Arredondo, who was in charge during the May 24 mass shooting in Uvalde, Texas, offered up the explanation in a new interview with the Texas Tribune where he defended law enforcement’s delayed response in taking down 18-year-old shooter Salvador Ramos.

Arredondo has been heavily scrutinized by critics, who believe lives might have been saved if police had acted quicker and gotten to the injured faster.

“Not a single responding officer ever hesitated, even for a moment, to put themselves at risk to save the children,” Arredondo told the newspaper. “We responded to the information that we had and had to adjust to whatever we faced.”

The door to the classroom that Ramos was in had a steel jamb and could not be kicked in, Arredondo told the paper. He spent more than an hour in the hallway trying dozens of keys.

“Each time I tried a key I was just praying,” Arredondo told the Texas Tribune. “The only thing that was important to me at this time was to save as many teachers and children as possible.”

In the first 40 minutes, Arredondo says he was waiting to receive the set of keys. During that time, he called for tactical gear and a sniper, while avoiding the doors, believing he might provoke Ramos to shoot. Dozens of keys finally arrived, but none of them opened the door.

“My mind was to get there as fast as possible, eliminate any threats, and protect the students and staff,” Arredondo said.

One hour and 17 minutes after Ramos started shooting murdering schoolchildren with an AR-15 style gun, police finally breached the door to the classroom and killed the gunman.

Arredondo’s account of the police response is not supported by the Texas Department of Public Safety, which is investigating the shooting and the subsequent police response.

DPS said Arredondo erroneously treated the shooting as a barricaded suspect incident, in which law enforcement negotiates with the gunman. Instead, he should have treated it like an active shooter situation, where the number one priority for officers is to stop the shooting by killing the gunman or taking him into custody.

“With the benefit of hindsight, where I’m sitting now, of course it was not the right decision. It was the wrong decision, period,” DPS Col. Steven McCraw said in a press briefing on May 27.

Arredondo tried to talk to the gunman through the wall, but there was no response, he told the newspaper.

The school district police chief also tried to justify his decision not to take his police radios into the school with him, believing he needed both hands to take down the shooter instead of holding the devices that might give away his position if the gunman heard them. Arredondo also did not have a bullet resistant vest, he told the paper.

“Our objective was to save as many lives as we could, and the extraction of the students from the classrooms by all that were involved saved over 500 of our Uvalde students and teachers before we gained access to the shooter and eliminated the threat,” Arredondo said.

But Arredondo’s decision not to take his radios in with him meant he did not know that students were calling 911 from inside the two classrooms the gunman targeted, begging for police to stop him.

He also said he didn’t consider himself the incident commander and thought another officer had taken control of organizing the different police agencies that responded.

“I didn’t issue any orders,” Arredondo said. “I called for assistance and asked for an extraction tool to open the door.”

Arredondo says he never told anyone to not breach the building, while simultaneously taking credit for telling officers to break windows from outside so that students in other classrooms could be evacuated.

Some of the US Border Patrol agents who responded to the school claim they ignored an order not to enter the classroom, reported the New York Times. Arredondo did not object when the team entered the room.

Arredondo did not speak out soon to avoid causing more grief to the families of the victims, he said. He has hired attorney George E. Hyde.

The Texas Department of Public Safety did not respond to The Post’s request for comment.