Federal agents documented a mystery “black triangle” flying at speed near a sensitive US site earlier this year, new Pentagon UFO files reveal.

The claimed sighting of the “matte black triangular shape of extremely low aspect ratio” took place at an undisclosed location near a “national security site” in the western United States in June, according to newly-released documents.

A FBI observation report said that two federal agents were stationed at an observation post several miles to the west of the location when they were alerted to the supposed sighting.