NEW YORK (NewsNation Now) — The carnage inflicted by the 9/11 hijackers 20 years ago was felt nowhere more keenly or cruelly than at the New York brokerage firm Cantor Fitzgerald, which occupied the 101st through 105th floors of One World Trade Center.

On the morning of Sept. 11, 2001, 659 Cantor employees were killed. That is every Cantor employee who came to work and was at the New York office at the time. Cantor CEO Howard Lutnick was not there because he was taking his son to his first day of kindergarten.

Somehow, Cantor resumed its business, along with the rest of Wall Street, just two days later. Rebuilding though, however, took years.

For five years, Cantor Fitzgerald paid a quarter of its profits to the families of the workers who were killed and paid for their health care for 10 years. Now, on each 9/11 anniversary, the firm invites celebrities to work the phones and 100% of the revenues on that day go to charity.

Along with his employees, Lutnick’s brother was also killed. He says every anniversary is painful — this one especially. But he, and the families of those lost, have found ways to cope.

“To be together with the families of my friends and all of those that we lost … we read their names, we talk about them, we remember them,” he said.

Now, Cantor Fitzgerald has 12,000 employees. Lutnick credits the employees he lost with the survival of the business.

“I always felt that my way of communicating with families was really on standing on my brother’s shoulders,” he said. “It is his being part of that group that allowed me to talk to them, allowed me to build a community with them, allowed me to rebuild this company.”

Lutnick believes one of the toughest things about every anniversary is rewatching the footage. Images difficult for an entire country to process became far worse to those who knew people inside the towers.

“When I see those pictures, it just causes my insides to just sort of rip apart because when that first plane hit that first tower, those are, those are my friends … that’s my brother, that’s my best friend,” he said.

He said one thing that does pick his spirit up on every anniversary is knowing his company has helped raise — on average — $12 million a year for 9/11-related charities.

“I can’t figure out how otherwise to spend my day. Otherwise, I’d stick my head under the pillow,” he said.