Stanton Friedman, the famed UFO researcher based in Fredericton, has died.

Friedman was returning from a speaking engagement in Columbus, Ohio, when he died suddenly at the Toronto Pearson Airport on Monday night, according to his family.

He was 84.

A nuclear physicist by training, Friedman had devoted his life to researching and investigating UFOs since the late 1960s.

He was credited with bringing the 1947 Roswell Incident — the famous incident that gave rise to theories about UFOs and a U.S. military coverup — back into the mainstream conversation.

Friedman "officially" retired last year but still booked speaking engagements "because he loved talking about UFOs," said his daughter, Melissa Friedman, who works for CBC News.

"Dad was curious about anything he didn't know about. He was always asking questions about how things worked.

Stanton Friedman, second from right, posing for a photo with his wife Marilyn, far right, his daughter Melissa Friedman and her husband David Parsons. (Submitted/Melissa Friedman)

"I think it's rare for someone to stay so engaged and curious and open-minded for a life that's that long."

She remembered her father as encouraging, proud and caring and said she was fortunate to have one last visit a week ago in Nova Scotia.

'He did his homework'

Friedman was an accomplished writer, publishing dozens of papers about UFOs and writing or co-writing several books. Three of those books were written in tandem with Kathleen Marden.

"He will be greatly missed," Marden, a UFO researcher, said in an interview from Florida.

Stanton Friedman, a leading authority on UFOs, is pictured taking part in a parade in McMinnville, Ore., in 2013. (CBC)

His qualifications, intelligence and diligence made him irreplaceable in the research field, she said.

"He did his homework," Marden said.

"He went further than most researchers in that he did on-site investigations. He went to actual physical archives to do his research. He was an outstanding researcher, highly intelligent and had a great sense of humour."

He was also a familiar face in documentaries, radio and television, including multiple appearances on Larry King Live, and lectured about UFOs for hundreds of colleges and professional groups across the United States, Canada and many other countries.

Friedman was also inducted into the UFO Hall of Fame in Roswell, N.M.

Stanton Friedman is survived by his wife of 44 years, Marilyn. (Submitted/Melissa Friedman)

Marden said he remained firm in his conviction that aliens exist and have visited our planet because he had "more than ample evidence."

"He doubted everything until he had the evidence," she said. "He was skeptical himself.

"Once he had the evidence and it was not just speculation — it was confirmatory evidence — he went with it."

'Trying to lift the laughter curtain'

In a 2011 interview with CBC News, Friedman said most people agree with him once they hear the evidence.

"Despite the false claims of a small group of nasty, noisy negativists, most people accept ET reality even though they think most others don't," he said.

"I check my audiences and find at the end of my lecture that about 10 per cent of the attendees have had a sighting. But 90 per cent didn't report it because of a fear of ridicule.

"I am trying to lift the laughter curtain."

His work was celebrated in New Brunswick, and the City of Fredericton declared Aug. 27, 2007, Stanton Friedman Day.

Friedman, who was born in New Jersey and had dual citizenship, lived in Fredericton with his wife of 44 years, Marilyn, mother to Melissa Friedman. He also had three children from a previous marriage.

Funeral arrangements have yet to be made.