Most Americans assumed he was joking.
"I come in peace," said President Barack Obama in March, 2012, after landing at Roswell, New Mexico, the infamous site of an alleged extraterrestrial spaceship crash landing.
Asked about the alien bodies reportedly recovered from the wreckage he added: "If I told you I would have to kill you... we're going to keep our secrets here."
But a controversial new book The Presidents And UFOs: A Secret History From FDR To Obama claims that a number of US leaders have known of evidence proving that aliens have visited Earth leaving spaceship debris, the occasional alien corpse and thousands of UFO sightings.
"Headquarters has come to the determination that the mystery airplanes are in fact not earthly and according to secret intelligence sources that are in all probability of interplanetary origin," wrote US Army chief of staff general George C Marshall to president Franklin D Roosevelt in March 1942, according to a recently leaked memo.
President Roosevelt replied urging the "finding of practical uses for the atomic secrets learned from the study of celestial devices".
And in another memo Roosevelt wrote of "coming to grips with the reality that our planet is not the only one harbouring intelligent life in the universe".
He added: "We will take every advantage of such wonders that have come to us."
The secret memos, whose authenticity is fiercely debated, are part of a mountain of evidence uncovered by the new book's author Larry Holcombe.
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