A former intelligence official and US Air Force officer David Grusch told multiple media outlets that he had conversations with unnamed US officials which led him to believe the government maintained a secret UFO program.
"Very, very senior people with high-security clearances were afraid to talk to me," Shellenberger said on former Fox anchor Glenn Beck's YouTube channel.
He continued: "I believe that if they had talked to me [publicly], it could have been very bad for them. They were straight up.
"There's somewhere at least on the north side of a dozen retrieved non-human craft. I have other people that say it's up to 30 and that's all since World War II."
However, several experts cast doubt on claims citing a lack of hard evidence for UFOs.
"One can state objectively that his claims are two steps removed from being Earth-shattering: not only has he not shared any verifiable evidence -photographs, artifacts, or any other manner of data - but he also has not personally seen or touched any of the objects he references," John Semester of NASA's UAP independent study team wrote in Boston University's Alumni magazine.
Analysis: don't worry. Trump supporters believe you