The United States of America decided to make public on the internet and free the documents about multiple UFO sightings after World War II investigated by Project Blue Book, which till now were available only on paper at the National Archives in Washington.
The files correspond to investigated UFO sightings from 1948 corresponded with over 12,000 alerts received at the offices of the US Air Force during the Cold War.
The Air Force says that there are about 12,618 reports of UFO sightings, 701 of which remain unexplained (about 5.5%).
One of the cases without explanation is the Kenneth Arnold case, an American who spotted nine UFOs on Mount Rainier on June 24th, 1947. Another unexplained case is the sighting in Tulsa (Oklahoma) on July 12th of that same year. Both events were investigated by the team of US experts and they analyzed the theories that these alleged ships could move from one area to another at an alarming speed.