SAN DIEGO -- Volunteers gather Friday morning to scour a 50-mile radius in the Alpine area to look for Brittany Hart, a Santee woman missing since May 24.

The volunteers include a private investigator and a group of Marines.

A registered sex offender is being held on suspicion of murdering the 24-year-old woman, but whose body has not been found, sheriff's deputies said.

Brittany Monet Hart, a Costco employee, was last seen buying gasoline at a Santee station, and detectives believe she is dead, Lt. Dennis Brugos said.

Robert Steven Carson, 40, was taken into custody on suspicion of murder after detectives found unspecified evidence in his Spring Valley home in the 1300 block of Marie Street, he said.