The testimony will be revealed in a new book, set to be published on Thursday, Express.co.uk can reveal.
The book, titled UFOs TODAY - 70 Years of Lies, Misinformation, and Government Cover-Up, has been penned by Dr Irena Scott, who gained a PhD in physiology from the University of Missouri.
Dr Scott said: "This book features the testimony of a former US Deputy Sheriff who was an alleged witness to the UFO crash at Roswell in 1947.
"This gentleman’s unique testimony will be published here for the first time."
Roswell has been at the heart of the UFO scene since in July 1947 the military sensationally announced in a press release it had found the remains of a crashed flying saucer in the desert nearby.
But the following day it retracted the statement, saying it was in fact a damaged US Air Force air balloon.
Witnesses later came forward to say there had been alien bodies within the "crashed craft", which along with the wreckage were then taken to the mysterious top-secret Area 51 military base in Nevada.
It has gone on to become the biggest UFO conspiracy theory on record.
Evidence seems to indicate bodies recovered in Roswell
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"This is significant because many of these people have died and those who are living are getting old.
"Thus, this research provides an historical record of the reasoning and opinions of those making an initial impact in this field."
Dr Scott went on to work for the Defence Intelligence Agency and the Aerospace Center in satellite photography.
She was also a volunteer astronomer at the Ohio State University Radio Observatory and has more than 30 years experience as a UFO field investigator with the Mutual UFO Network (MUFON).