Former US Air Force photo technician Karl Wolfe, who claimed that he saw secret NASA photos of alien structure on the surface of the moon, has died at the age of 74.

According to Fox News, the famous UFO whistleblower died in a bicycle accident in Lansing, New York, where he was fatally hit by a tractor-trailer. Wolfe was rushed to a nearby hospital, but he later died from the injuries.

Wolfe shot to fame in 2001 when he alleged that the US space agency is intentionally covering up the existence of alien structure on the dark side of the moon. The former airman made his controversial claims at an event in Washington, D.C. organized by Steven Greer, a Ufologist who founded the Center for the Study of Extraterrestrial Intelligence and the Disclosure Project, which calls for the disclosure of allegedly secret UFO information.

Wolfe spoke out about the alleged alien base on the lunar surface decades after leaving his position at Langley Field, Virginia, where he once worked for the US Air Force Director of Intelligence at the Headquarters Tactical Air Command, Technical Group, which served as a center for data and pictures from the NASA Lunar Orbiter Project.

According to the Express, Wolfe claimed that he was one of only two technicians at Langley who had enough security clearance to work with high-tech photographic equipment that processed information from military intelligence hardware. Wolfe related that he was at the color lab one day when his boss told him they were having a problem with some equipment.

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“An airman second class was in the darkroom at that time – I was also an airman second class. About 30 minutes into the process he said to me in a very distressed way, ‘By the way we’ve discovered a base on the backside of the moon,'” Wolfe said in a YouTube video of his testimony, as quoted by the International Business Times.

Wolfe said that his fellow Air Force man showed him photos of what he claimed to be alien structures on the moon. He said that these were not created naturally by meteors or ancient collisions of heavenly bodies. Wolfe said that the towers, domes, and mushroom-shaped structures on the picture were likely built by intelligent beings.

Police authorities have launched an investigation into the death of Wolfe. No charges have so far been filed against the driver of the vehicle that hit him.