Credit: ABC News

Crouched in fear underneath her desk, Sally Cox, the Sandy Hook Elementary School nurse, watched gunman Adam Lanza through a small hole holding computer wires as he walked into the school's office and faced her hiding place.

"I could see him from the knees down, 20 feet away, his boots were facing my desk," Cox told "Good Morning America" this morning. "It was seconds… and then he turned and walked out and I heard the door close."

On the morning of Dec. 14, Cox said she first knew something was going on inside the Newtown, Conn., school when she heard a series of "loud popping noises" outside of the office, but she didn't process that something was wrong until the school secretary, who was sitting at a desk outside her nurse's office, called her name.

"Your mind doesn't think that it could be [gunfire]," she said. "But when the secretary called out to me with terror in her voice, it just told me something terrible was happening."

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After the door closed, Cox said the secretary raced into her office, shut the door and hid under her desk. Together, they pulled the phone off the desk and dialed 911. They then raced into a supply closet and held the door closed. Cox said they remained there for close to four hours.

"We were petrified," Cox said. "We were listening, and couldn't hear a whole lot, and heard screaming and the gunshots."

Lanza would go on to kill 20 first graders and six adults inside the school, before turning the gun on himself, according to police.

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When the police made rounds, looking for survivors, they came across Cox and the secretary still in the office, and escorted them out of the building. Cox, who had tested every kid's hearing and vision in Sandy Hook, said the police shielded them from the horrific carnage in the hallway.

"They told us to close our eyes. They said, 'Close your eyes,'" she said.

Cox said at first she didn't want to know about the victims, but eventually came to learn their names with the rest of the country when police announced the list on Saturday. Cox said she wasn't sure if she will ever return to Sandy Hook.

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