New York Police Officer John Perry was putting in his retirement papers at police headquarters when he heard the call for a fire at the World Trade Center.

He raced to the scene to try to help people in need, and he never came out.

Fred Morrone, the Chief of the Port Authority Police, heard the call too. He could have easily stayed in his office in Jersey City. But Chief Morrone raced to the scene too, and started climbing the stairs to get people out.

“Witnesses said he would stop to reassure them that everything was going to be alright, that they'd get out safely," said Craig Floyd of the National Law Enforcement Memorial Fund.

The chief kept climbing.

“When that tower collapsed, his wife and son, sitting at home, when they say that first tower collapse, they said, my father, my husband is gone,” said Floyd.

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Seventy-two law enforcement officers died on 9/11. It was the deadliest day ever for American police.

At the National Law Enforcement Memorial, on a beautiful crystal blue day like that horrible day 16 years ago, they read the names of each officer, and they seemed to go on forever.

“When you meet the families, hear the stories behind the names, you feel like you did know the officers,” said Floyd.

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Seventy-two names of 9/11 heroes share a wall with 21,111 other heroes, police officers answering the call to danger, even today.

On average, a police officer is killed answering the call every 58 hours in the U.S. That's about 151 sons and daughters, mothers and fathers every year.