Eventually, a janitor provided six keys. Arredondo tried each on a door adjacent to the room where the gunman was, but it didn’t open.

Later, another key ring with between 20 and 30 keys was brought to Arredondo.

“I was praying one of them was going to open up the door each time I tried a key,” Arredondo said in an interview.

None did.

Eventually, the officers on the north side of the hallway called Arredondo’s cellphone and told him they had gotten a key that could open the door.

The officers on the north side of the hallway formed a group of mixed law enforcement agencies, including U.S. Border Patrol, to enter the classroom and take down the shooter, Arredondo said.