In making the announcement, District Attorney Bonnie Dumanis released no details about a possible motive or evidence involved in the arrest of Jessica Lynn Lopez in the death of 22-year-old Brittany Dawn Killgore.
Investigators have said Killgore was last seen with another Marine, Louis Ray Perez, who was arrested on a weapons charge.
Dumanis said Thursday that Perez remains a person of interest in the Killgore case but she did not elaborate.
Authorities have not said if the three people knew each other or whether the stolen weapon was related to Killgore's death.
Perez's attorney Jeffrey Reichert said his client has been in the military for 16 years, has no criminal history, is married and has lived in the San Diego area for 10 years.
Killgore, who was married to a Marine deployed to Afghanistan, disappeared last week after she filed for divorce from her husband.
Her body was found near Lake Skinner in Riverside County about 25 miles northeast of her modest apartment complex in Fallbrook, a town of 38,000 people known for its avocado orchards, rolling hills and proximity to Camp Pendleton.
Neighbors were stunned when coroner investigators identified the body of Killgore.
"It's very bizarre. I wish I knew the end to the story. Where are the leads?" said Navy Petty Officer Ashley McMurray, who has lived at the complex of yellow stucco homes on Ammunition Road for 11 months.
Perez is being held on $500,000 on bond after pleading not guilty to stealing an assault weapon. Authorities say he took the weapon from a man in early February.
Lopez, who lives at a home a mile from the Killgore residence, was arrested Tuesday. She was scheduled to be arraigned Thursday.
Lance Cpl. Cory Killgore was returning to California. Authorities say he is not a suspect.
The Killgores' apartment is a five-minute walk from an entrance to Camp Pendleton. The complex -- like others around it -- is full of Marines and their families who tend to be short-term residents and often don't get to know each other. Residents are accustomed to the buzz of helicopters and bomb blasts at the neighboring base.
Patricia Pagdilao said the Killgores were already living in the upstairs apartment when she moved in with her Marine husband a year ago. Cory Killgore visited with her husband to talk over beers and seemed happy with his marriage, she said.
After Cory Killgore went to Afghanistan about seven months ago, Brittany Killgore went out often with friends.
"The only time I ever saw her was with friends, either on the way out or on the way home," Pagdilao, 20, said.
The couple from Missouri was married in July 2010. Brittany Killgore cited "irreconcilable differences" in her divorce filing, which gave no other details.
Her cellphone was found in the Gaslamp District of downtown San Diego.
Darryl J. Wrest of Rolla, Mo., confirmed Brittany Killgore is his daughter but declined to comment further. Killgore's other family members also declined to comment.