Interdimensional beings

The Aliens arrived on Earth about 5000 years ago. They were humanoid but their heads were elongated. Instead of bones, their skeletons were made of pure crystal and had magnetic qualities. According to Harold Oxley, they were multidimensional beings not from space, but from the 'space between spaces', and traveled with flying saucers.

According to the murals found in the entrance of Akator, they taught the Mayans agriculture, technology and writing. Eventually they were worshiped as gods. Indy noticed that the Mayans had the custom to wrap their children's heads to cause an elongated shape, in order to 'honor the gods'.

13 of those aliens were situated in the Temple of Akator, perhaps as a kind of ruling council. For some reason they were left dead there as skeletons. Centuries later, Francisco de Orellana found the Temple and removed the skull of one such skeleton.

The aliens seem to kept visiting Earth. One of their vehicles crushed on Roswell in 1946. Indiana Jones was part of the team ordered to recover the remains of a 'mutilated' body in absolute secrecy. The body was eventually kept in Hangar 51.