San Diego Superior Court spokeswoman Karen Dalton says Louis Ray Perez will be charged Wednesday afternoon in Vista, Calif.
Sheriff's Department officials say the 45-year-old Perez was the last person seen with Brittany Dawn Killgore. Investigators found a body in neighboring Riverside County on Tuesday but its identity has not been confirmed.
Perez was arrested Sunday for possessing a stolen AR-15 rifle. The Marine Corps does not have AR-15 rifles and the source of the weapon is unclear.
He's jailed on $500,000 bail.