Ken Haskell was off-duty on Sept. 11, 2001. But when a Boeing 767 struck the north tower that morning, the firefighter shot over to his station before hurrying to the World Trade Center.

"As soon as the plane hit, I knew it was a terrorist attack," he told Fox News.

Timmy and Tommy Haskell, Ken's brothers and fellow New York firefighters, also rushed to the twin towers. Each sibling came from a different part of the city and arrived separately.

The trio was drawn toward public service at an early age. They wanted to follow in the footsteps of their father, who served as a Marine and as a New York City firefighter.

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Tommy and Timmy Haskell dress in their uniforms to attend the St. Patrick's Day parade in New York City. (Courtesy Ken Haskell )

"We all just always had an inclination to help other people," Ken said. "I think that’s really what drove us to that job." 

A second plane struck the south tower at about 9 a.m. and collapsed by 10, before Ken could make it to the crash site.

When the north tower fell just before 10:30, he was driving over the Manhattan Bridge. He turned to a colleague and said, "We all know someone who just died."

"I didn’t know about my brothers at that time," he told Fox News.

The Sept. 11 terrorist attacks — 21 years ago Sunday — killed nearly 3,000 people, including almost 350 firefighters. Timmy and Tommy were among them. Timmy's body was found four days later. Tommy's remains were never identified.

"If those planes hit again tomorrow, they would be the first guys running back in there," Ken told Fox News. "There's no doubt in my mind."

"But I take solace in the fact that they died doing what they loved," he added.

Aftermath of terrorist attacks on September 11, 2001 in New York City

Firefighter walks through the rubble of the World Trade Center following the 9/11 terrorist attacks. (Todd Maisel/NY Daily News via Getty Images)

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'That's something to be heralded'

As much as Ken misses his brothers, he emphasized how proud he was of their final heroic moments, which fellow FDNY firefighters at the towers recounted for him.

Timmy, like Ken, was off-duty on Sept. 11, but rushed from his home in downtown Manhattan anyway. He and a fellow Squad 18 firefighter sped up a stairway in the north tower as soon as they arrived.

"They were able to get to about the 30th floor and came across somebody" in medical distress, Ken told Fox News. "They stopped and provided first aid."

The pair determined that they needed to help the man down the stairs. Timmy's colleague went to check another staircase, but Timmy was gone when he returned. A Port Authority police officer who arrived while the firefighter was gone said Timmy went farther up to help someone else.

"He made a decision, and nothing surprised me about the decision he made," Ken told Fox News. "Somebody else needed help, and he was gonna do it." 

Timmy's colleague and the officer instead helped the man down. The three safely made it down to the lobby. The building began collapsing and created a force that propelled them out of the building, the firefighter told Ken.

Timmy, meanwhile, made it high up in the building before it crumbled.

"That left him pretty high in the rubble pile," Ken said. "We found him four days later."

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FDNY firefighter Timmy Haskell

Timmy Haskell, of Squad 18, died after running up a staircase in the north tower of the World Trade Center on Sept. 11, 2001.   (Courtesy Ken Haskell)

Ken was at Ground Zero every day, on- and off-duty in the weeks following the collapse. He was at the south tower recovering remains when Timmy was identified. 

"I saw one of the guys from the company there, and he just had this look on his face," Ken said. "I knew. I just said ‘which one?’"

Ken drove to Seaford, Long Island, where his family was gathered around the television, hoping for good news.

"That was a difficult moment having to tell [my mother]," Ken told Fox News. Then she asked about Tommy.

"I said ‘well, we're still looking for him,'" the firefighter recounted.

Tommy and Ken Haskell with their mom

Ken and Tommy Haskell with their mother in their FDNY dress uniforms. (Courtesy Ken Haskell )

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Tommy's company, Ladder 132, was just behind Ladder 113 in the south tower, Ken was told.

"All the guys from 113 survived and all the guys from 132, few more seconds, they possibly could have survived, but they were all lost," Ken said.

Tommy's three children filled his casket with mementos as his remains were never identified.

Ken said Tommy, as a captain, made it a priority to be "mindful of everyone else's well-being around him."

"I knew that day he would be the last guy out regardless of whatever the circumstance was," Ken added.

He said if he could see his brothers again he would give them both big hugs. 

Haskell brothers names honored on 9/11 memorial in New York

FDNY firefighters Thomas Theodore Haskell Jr. and Timothy Shawn Haskell honored at the National 9/11 Memorial and Museum in New York.  (Photo by ROBERTO SCHMIDT/AFP via Getty Images))

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"I'm grateful to know what they did, and I'm proud of what they did," Ken told Fox News. "People made decisions, put themselves at risk, and that's something to be heralded."

"I miss them terribly," he continued. "We did everything together. We did the same careers together. I cherish those memories."

"I want to live the best life I can through their example," Ken said. "I know I'm going to see them again. That's what gets me through."