The disclosure came after Louis Ray Perez, 45, pleaded not guilty to first-degree murder in the death of Brittany Killgore.
A woman who once lived with Perez - 25-year-old Jessica Lynn Lopez - has also pleaded not guilty to murder.
The brief arraignment offered the most details yet of what authorities believe happened to Killgore, whose death has shaken many at Camp Pendleton and in neighboring Fallbrook.
Killgore sent a text message to a friend pleading for help after making plans to go out with Perez and another woman on April 13, said Patrick Espinoza, a San Diego County prosecutor. The other woman, who was not identified in court, didn't show up for the date, he said.
"She sent a text saying, 'Help,'" Espinoza said about Killgore. "She was missing from that point on."
Perez told investigators he dropped off Killgore in downtown San Diego's Gaslamp District, but he actually stayed in Fallbrook, about 60 miles north, authorities said.
No other details, including a possible motive, were disclosed.
Killgore's body was found in brush near Lake Skinner, about 25 miles north of Fallbrook, on April 17, hours after paramedics were called to a San Diego motel where they found Lopez with self-inflicted cuts and a suicide note, authorities say.
Perez's attorney, Jeff Reichert, said the suicide note clearly shows that Lopez killed Killgore and that his client was not involved. Superior Court Judge Kimberlee Lagotta ordered the note to be sealed.
Espinoza did not identify the weapon he said was found in Perez's car or say if it was used in Killgore's death.
Perez, who has been a Marine for 16 years, has also pleaded not guilty to stealing an assault weapon. The prosecutor said Perez stole the weapon and a motorcycle from a trailer at Camp Pendleton.
Killgore died less than a week after filing for divorce from Lance Cpl. Cory Killgore, a Marine who was in Afghanistan when she disappeared. She cited irreconcilable differences in her divorce filing, which gave no other details.
Cory Killgore has not been named as a suspect or person of interest in the killing. The couple married in July 2010.
The couple lived in Fallbrook, a town of 38,000 people, in an apartment complex a mile from the home of Lopez.
Perez and Lopez lived together at one time, said Espinoza, who did not reveal when or where.
The judge set bail at $3 million for each defendant and scheduled a preliminary hearing for May 30.
