Vanity Fair has the clues and the reader is left to connect the dots:

Film is set in 1957 (ten years after crash at Roswell), was shot in New Mexico, and in contrast to previous Indy Films that pay tribute to 1930s serials, this one takes inspiration from 1950s B movie science fiction. The villains are the Cold War Russians and the race is to recover a “Crystal Skull,” most likely an alien artifact with vast technological power.

Confirmation comes May 22nd with world wide release.