The 9/11 attacks were aimed at the financial heart of America. Many Wall Street workers whose offices were in the Twin Towers can recall every moment of that day, including a security officer whose quick thinking helped save more than 100 people. NY1's Diane King Hall spoke to some of them.

15 years ago, New York Stock Exchange (NYSE) staff worked on the 28th, 29th and 30th floors of the World Trade Center's South Tower, and traders worked in companies throughout the building.

"I got there around 7, 7:15 in the morning and was there until about 8:15, 8:20 because I had an 8:30 meeting here at the New York Stock Exchange," said Jonathan Corpina, the senior managing partner of Meridian Equity Partners.

That meeting on Wall Street was Corpina's saving grace.

NYSE security officer Oliver Howard was on the 30th floor when he heard an explosion: the first plane had hit the North Tower.

"I saw the entire top of World Trade Center 1 on fire," Howard said. "I couldn't believe it."

A voice on the loudspeaker urged everyone to return to their offices, and that the problem was in the other tower.

"I said to myself, 'I gotta get everybody off the floors. We have to leave the building,'" Howard recalled. "So I went quickly to every floor that we had...and told everybody that we have to leave."

Everyone in the Exchange's offices — some 130 people — were brought to safety, thanks to Howard's quick thinking.

Corpina, who was down the street, recalls a friend saying that the towers were going to come down.

"I said, 'You are absolutely crazy. He said, 'No, no we should really start walking Uptown,'" Corpina said. "I said, 'No, that makes no sense.'"

But he was persuaded to leave.

Soon, the towers collapsed, and the Exchange reacted quickly

In a near unprecedented act, the Big Board shut its doors that day and extended its closing to four straight days, the longest that the NYSE had been shut since the 1930s.

The Exchange reopened Sept. 17. The NYSE honored Howard that day, letting him ring the closing bell.

Today, Corpina still can't imagine working at the Trade Center site.

"I still haven't been over to that site yet," he said.

Friday, the NYSE remembered those who died with a moment of silence, as they have every year since 9/11.