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After half a century of investigation, a former nuclear physicist -- who worked on fission and fusion rockets for companies like Westinghouse and Aerojet General Nucleonics -- is convinced that not only are UFOs real, the government has known about them since 1947.

"Some UFOs are intelligently controlled extraterrestrial spacecraft," Stanton Friedman told AOL News, calling the vast cover-up of their existence "the biggest story of the millennium."

Employed for 14 years as a nuclear physicist for companies like General Electric, General Motors, Westinghouse and Aerojet General Nucleonics, he worked on highly classified programs involving nuclear aircraft, fission and fusion rockets.

In 1958, UFOs caught his attention, and Friedman has since lectured about this subject at more than 700 colleges and professional groups in all 50 states and around the world.

"After 53 years of investigation, I'm convinced we're dealing here with a cosmic Watergate," he told AOL News. "That means a few people within major governments have known since at least 1947 that some UFOs are alien spacecraft."

In Friedman's new book, "Science Was Wrong," co-authored with Kathleen Marden, he wrote, "There's been no shortage of strong, negative proclamations from debunking groups and individuals who refuse to examine the evidence ... to support the notion that some UFOs are of extraterrestrial origin."

While some scientists through the years have quietly suggested Earth has been visited by ETs, Friedman is the most outspoken. He's especially irked by the attitude of scientists who use radio and optical telescopes in the search for extraterrestrial intelligence, such as the SETI Institute in California.

"What really bothers me is that the SETI people will tell you there is no evidence for UFOs. Well, they certainly don't reference any, so there must not be any, right? Wrong!"

For more, see the full story on AOL News.