(CBS) SAN JOSE - Police are reviewing sonar images taken from local reservoirs as the search continues for missing California teen Sierra LaMar.
CBS San Francisco reports that LaMar, 15, was last seen the morning of March 16, on her way to a school bus stop.
Earlier this week, sheriff's officials took sonar images of Chesbro and Uvas reservoirs. The department plans to send out divers later in the week to investigate any suspicious objects they may find.
Volunteers gathered Wednesday for another day of searching at Burnett Elementary School. According to search director Brian Miller, volunteers are looking in areas previously unsearched.
LaMar has been missing for over three weeks, but her mother Marlene said she is trying to stay optimistic.
"I'm not going to give up," said Marlene LaMar. "I am determined, and the determination is what's giving me the energy to continue. The public is as well. People are coming back for a third and fourth time. I'm truly astounded."
KCBS reports that about 3,500 people have helped in the search since LaMar disappeared.
There is currently no named person of interest in the case and police are still treating it as a missing person case.
Anyone with information about Sierra or her whereabouts is asked to contact Santa Clara County Communications at (408) 299-2311. During business hours, tipsters can call sheriff's investigators at (408) 808-4500 or the anonymous tip line at (408) 808-4431.