Investigators said they believe there is a strong likelihood she is the victim of foul play.
Louis Perez, 45, is believed to be one of the last people seen with her and has been arrested on unrelated charges, according to officials.
He has been questioned regarding Killgore's disappearance, but detectives said he was uncooperative.
Perez is an active duty Marine based at Camp Pendleton and is currently in San Diego Central Jail for allegedly stealing an AR-15 assault rifle.
Investigators are now classifying Perez as a "person of interest."
"He is uncooperative and not completely forthcoming about his knowledge we believe," said Lt. Larry Nesbit.
Neighbors said Killgore's husband, Cory, was on deployment during the incident.
He posted on his Facebook page on Sunday asking for prayers for his family.
The couple filed for divorce on April 10, according to court documents.
Sheriffs said the Red Cross is now helping to fly Cory home from Afghanistan and said her family from out of state is en route to Fallbrook as well.
A car parked at her residence had explicit language scribbled in the dust of the car windows.
Investigators said the comments were not there yesterday.
Killgore's neighbor, Patricia Pagdilao, said Killgore is often out late at night but that she's always with friends.
"Usually I see her you know every night coming through here when she invites a couple of friends over but the last time I saw her was last week," Pagdilao said.
Investigators searched in De Luz Canyon near Fallbrook today based on information they received. They are still looking for Killgore.
Killgore is described as 5-foot-8 with brown hair, brown eyes and weighs 130 pounds.
Killgore was last seen wearing a dark purple evening gown with purple floral glitter patterns.
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