A man who spent decades investigating UFOs, convinced they were real, has died without finding solid proof.

Stanton Friedman was a nuclear physicist with a background working on aircraft, rockets and power plants. But he had a keen interest in aliens and UFOs, and in the 1970s led the first civilian investigation into the 1947 Roswell incident.

That work culminated in 1997 book Crash at Corona: The Definitive Story of The Roswell Incident, which included evidence from more than 100 witnesses to the world's most infamous alleged UFO crash.

According to Friedman, two alien craft crashed that day in July, 1947. The official story has always been that it was a top-secret balloon.

Friedman called the supposed cover-up a "cosmic Watergate".

While he was convinced UFOs were being flown by real alien visitors, he died without ever seeing one himself, the Daily Mail reports. Most of the evidence he collected was based on US government documents, the Globe and Mail reports.

"For me he will always be the greatest ufologist of all time," UFO Truth Magazine editor Gary Heseltine told the Mail.

"He never wavered in his belief that some UFOs were [extra-terrestrial] in origin. I mourn the passing of a truly legendary man."

Friedman was 84. His family did not disclose the cause.

Newshub.