A retired U.S. Army Colonel has said continuing to hide information about UFOs could have "catastrophic" consequences for America, amid new claims that government officials agreed to hold back top-secret research 20 years ago.

Colonel Karl E. Nell called on a Stanford University conference for a "campaign plan" that would force greater transparency and a "Manhattan project" to reverse engineer recovered UFOs or Unidentified Anomalous Phenomena (UAP), the Daily Mail reported on Tuesday.

Washington insiders also heard how in 2004, a CIA thinktank, the Defence Intelligence Agency, and the Pentagon, broadly agreed that information about UFOs should not be declassified, deeming the societal risks too great.

That has been the prevailing sentiment among the powers that be for 50 plus years and will most certainly not change.

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