Sandy Hook Elementary School nurse Sarah Cox was on the phone with her daughter, Bonnie, when she heard the strange, loud popping noise coming from outside.
“What’s that?” Cox asked.
She was calling to Barbara Halstead, a secretary who worked in the main office at the school’s entrance, which is adjacent to the infirmary.
When Halstead responded she didn’t know, Cox abruptly ended the call with Bonnie and prepared to investigate.
Then she heard Halstead call her name.
“The way she said my name, in a terrified tone of voice, I knew it was bad,” said Cox, a 60-year-old grandmother who has worked at the school for the last 15 years.
Cox, known as Sally to friends and co-workers, took no chances.
“I just dove under my computer desk,” she told The Post in an interview at her home in Newtown, a day after Adam Lanza stormed into her school and gunned down 20 children and six staffers.
Within seconds, the popping sounds stopped as quickly as they had started, and Cox held her breath when she heard someone open her office door.
Through the openings of the desk, she saw a pair of legs from the knees down wearing dark clothing and boots.
“I could see by the position of the feet that they were facing my desk,” Cox said, describing her brush with Lanza, 20, who had earlier killed his mother, Nancy, before unleashing his terror on the school.
Cox said he was no more than 20 feet away from her.
“The person was there just a few seconds,” Cox said. “I was frozen in fear.”
Cox crouched paralyzed under her desk.
“He turned around and walked out, and as soon as the door closed I heard more popping,” Cox recalled.
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In the main office, Halstead had also been hiding under a desk.
“By the grace of God, there were no children in the office, and in my office,” Cox said.
Halstead then made a dash for the infirmary and dove under Cox’s desk. She grabbed the office phone and immediately called 911.
“Help, we have a shooter,” Halstead said into the phone. “It’s the school. Get help right away.”
Seconds later, the two terrified women ran into the first-aid supply closet to hide.
“I could hear popping and screaming, and it was horrible, knowing I couldn’t do anything,” Cox said.
They remained in the closet for more than an hour.
“We were praying, ‘Please stop, please stop,’ ” she said. “We had hoped that nobody got hurt. That maybe he was just spraying bullets around.”
At 11:15 a.m., they opened the closet door a crack. From her office window, Cox looked out into the courtyard where she saw several men wearing fatigues and toting weapons.
She said she didn’t know whether they were SWAT team members — or the attackers.
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Neither Halstead nor Cox had their cellphones with them.
She and Halstead had no idea what was going on, but they could hear helicopters overhead, people on the roof of the school, shouting and yelling. At one point, someone jiggled her office door, but did not call out.
It would be another two hours, at 1:15 p.m., before the two women summoned the courage to open Cox’s office door.
“I just got brave enough to open up the door to a slit,” she said.
That’s when she saw police, who eventually escorted her and Halstead out of the school.
“They didn’t really tell me too much,” Cox said. “When they walked me out of the school, they told me to close my eyes.”
When she learned Lanza’s identity, she could not recall him being a student at the school.
Cox, who takes pride in knowing all of the 580 pupils who attended Sandy Hook Elementary, said she didn’t yet know the names of all of the victims of Friday’s violence.
“I haven’t asked,” she said. “I haven’t wanted to know.”