When the North Tower collapse on 9/11, Thomas Gambino III knew in his gut that his father, FDNY Firefighter Thomas Gambino Jr., was dead.

At his father’s funeral, he knew what he’d be doing the rest of his life.

The younger Gambino, who was 20 at the time, is one of more than 75 sons and daughters of the 343 FDNY members lost on 9/11 — or later succumbed to Ground Zero-related diseases — who went on to become firefighters or medics.

“I was actually supposed to go surfing with my friends and for whatever reason we had a late night and I didn’t get up that morning,” Gambino III, 43, told The Post.

“I could hear the phone ringing in the kitchen of the house. I still lived at home. And it just kept ringing and ringing. I was like all right, I’m getting up.”

It was his friends calling to tell him to turn on his TV.

“When the buildings collapsed, the South Tower first, it was gut wrenching,” said Gambino III. “Not just because of the overall magnitude of it. I had a really strong suspicion that my dad was in there pretty much right away.”

Later that night, his family got confirmation that his dad, who worked at Rescue 3 in the Bronx, “was in the pile somewhere” under the North Tower.

For years before he had thought about joining the department but never fully committed to it. Instead he joined a band, and got his two-year degree at Suffolk County Community College.

“The pivotal moment was really arriving at the funeral,” said Gambino III. “It struck something in me hearing the bagpipes and seeing these guys line up.

“Though I was completely grief stricken and overwhelmed by my pain, I knew at that point that I was going to do this,” added Gambino III, who’s been with the FDNY since 2004 and works at Ladder 126 in Jamaica/South Ozone Park in Queens.

The NYPD lost 23 officers in the 9/11 terror attacks, and Detective Jillian Suarez is one of six known to have followed their Finest parents into the force.

Her dad, Roman Suarez, a 45-year-old NYPD transit cop, saved two women, one an asthmatic and the other, seven-months-pregnant, from the North Tower before dying in the terror attacks. He was posthumously awarded the NYPD’s Medal of Honor. 

Her father’s death “made Jillian want to go into actual law enforcement, and honor her father and wear his badge,” her mom, Carmen Suarez, recalled.

Jillian, who joined the NYPD’s ranks in 2018, is now a detective in the department’s crime scene unit, working all over the five boroughs, her mother said. 

“I am absolutely beyond proud of her,” Carmen said. “I get scared, of course. There’s a lot of danger out there in law enforcement, and sadly, some people forget that it’s not just a blue uniform — behind that uniform is a person just like you and me,”

Brothers Joe and James Vigiano both followed their dad, NYPD Detective Joseph Vigiano Sr. — and their mom — to become Big Apple cops.

James was 6 years old when his father died on 9/11 trying to rescue victims as a member of the department’s Emergency Services Unit 2.

The now 29-year-old son remembers being in school, seeing teachers in the hallway crying, then being led into the cafeteria — where he saw his brother and other children of cops and firefighters.

It wasn’t until later that day that his mom, now-retired Police Officer Kathy Vigiano, told James and Joe that their dad wouldn’t be coming home, James said in a video produced by the NYPD.

“Honestly, I’ve wanted to be a police officer since I could talk,” he said. “I had two dreams: I wanted to be a Marine and I wanted to be a police officer and I’ve done both.”

Today, James works in the 75th Precinct in East New York, the same precinct where his mother once served. Joe, now 31, is a member of the NYPD’s Emergency Service Squad 7 in Brooklyn. 

The third Vigiano brother, U.S. Marine John Vigiano, who is stationed in Hawaii, also plans to become a police officer, his mother told The Post. Whether it’s in Gotham is still up in the air.

“He keeps going back and forth,” Kathy said, explaining that he plans to either join the NYPD or the Suffolk County Police Department.

“They were so young that we never talked about what they might do when they grew up,” she said. “But I think he would be very proud.”