What is the main conflict in Talking God by Tony Hillerman?

Talking God is a gripping mystery by Tony Hillerman, set in the American Southwest and Washington, DC, and involving two of Hillerman's famous detectives, Lieutenant Joe Leaphorn and Office Jim Chee of the Navajo Tribal Police. Leaphorn and Chee each investigate a mystery that leads them to the nation's capitol, where they join forces to uncover a terrorist plot involving Indian artifacts and a priceless golden jewel-encrusted Inca mask. Together, the two heroes defuse a bomb at a crowded Smithsonian exhibit and return the Navajo Yeibichai or Talking God mask to its rightful owners.