San Diego County Sheriff's Capt. Duncan Fraser told reporters Tuesday Jessica Lynn Lopez, 27, was taken into custody Tuesday morning. The body, found in a rural area of Riverside County, has not yet been positively identified, The San Diego Union-Tribune reported.
Brittany Killgore, 22, of Fallbrook, Calif., who is married to a Marine deployed in Afghanistan, was last seen Friday.
San Diego paramedics responding to a call at the hotel where Lopez was arrested reported finding a woman suffering from apparently self-inflicted cuts, the newspaper said. Lopez -- who lives about a half mile from Killgore -- was under guard at University of California-San Diego Medical Center.
A Marine stationed at Camp Pendleton, Calif., Louis Ray Perez, 45, has been in jail since Sunday, when he was arrested on unrelated weapons charges. He is expected to be arraigned Wednesday, the newspaper said.
Fraser said Perez was "being actively investigated for this disappearance."
Killgore has not been seen since Friday. Her cellphone was found by trolley security in San Diego's Gaslamp Quarter but detectives said Monday they have reason to believe she never left the Fallbrook area.
Detectives said Perez was uncooperative when they questioned him. He was being held in jail in lieu of $500,000 bail.
Killgore's husband, Cory, was returning to California. Court records indicated Brittany Killgore filed for divorce last week, three days before she disappeared.