SANTA ANA, Calif. -- Investigators shifted their focus to Los Angeles Monday in the search for a 19-year-old San Diego State University student after recovering items they believe belong to a man sought for questioning in the honor student's disappearance.

Teams of investigators from the Orange County sheriff's and Los Angeles Police departments went to 3617 Faris Drive in the Palms area of Los Angeles, where Donna Jou was last seen at a party on June 23, said Jim Amormino of the Orange County Sheriff's Department.

Jou, who was taking community college courses in Orange County over the summer, never returned home after the party.

Investigators believe that a registered sex offender, John Steven Burgess, picked her up near her home in Rancho Santa Margarita on his black motorcycle, and the two went to Los Angeles, Amormino said.

On Sunday, a black plastic tool box similar to one on the back of a Ford Ranger pick-up truck owned by Burgess, who also goes by the name of Sinjin Stevens, was found about 1.5 miles from the house where Jou was last seen, Amormino said.

Inside the box was a "SINJIN" personalized license plate, a black motorcycle helmet and other items investigators are not identifying, Amormino said.

The homicide detectives consider the disappearance of Jou as "a critical missing" case, with the young woman "possibly a victim of foul play," he said.

Jou was last seen near her Rancho Santa Margarita home about 5:30 p.m. June 23 climbing onto the back of a man's black motorcycle. Amormino said last week they believe the man was Burgess.

On the night of the party, one of Jou's friends in San Diego received a text message from her, saying she had locked herself in a bathroom because things were out of control and a guy was giving her the creeps, Amormino said.

The next day, her mother got a text message saying Jou was coming home, but she never arrived.

It was unusual for the young woman to text instead of call her mother and investigators question whether she was the one who sent the message, which referred to "Mommy," a term Jou did not use, Amormino said.

Her mother reported her daughter missing the day after the party, but because she is an adult and could be gone voluntarily, authorities did not immediately begin to search for her, Amormino said.

Since then, investigators have determined she has not used her cell phone or credit cards, which has also raised concerns.

Jou's family turned over a picture of a man whose name they thought was Kyle, who they believed communicated with Jou on the Web site craigslist under the code-name "Skinny Puppy." But authorities spoke to the man and determined he had nothing to do with Jou's disappearance, Amormino said.

Amormino said the family apparently took a photo off the Internet site, and that the man had no connection with either Burgess nor Jou.

Burgess was linked to Jou through witnesses who were at the party at his house, Amormino said.

His black 1981 Yamaha motorcycle, with the brand name spelled out in green lettering, is missing, acccording to Amormino, who said Burgess may also be driving his 1998 blue Ford Ranger truck.

Jou may have had a blue Jansport back pack with her, he said. The SDSU student, who was taking summer classes at Rancho Santiago Community College in Orange, has a 4.0 grade point average, but is not especially streetwise, according to her family.