ESCONDIDO, Calif. -- Police personnel and civilian volunteers traipsed through brushy gullies and muddy wetlands again Friday in hopes of finding any sign of a 14-year-old North County girl who went missing three weeks ago.

The continuing search for Escondido High School student Amber Dubois was bolstered by help from two high-profile victims of child abduction but turned up no clues to the whereabouts of the brown-haired, blue-eyed teen.

On hand to help draw attention to the increasingly urgent effort were Marc Klaas, who became a well-known child advocate after his 12-year-old daughter, Polly, was kidnapped and murdered in 1993, and Brenda van Dam, whose 7-year-old daughter, Danielle, was abducted and slain by a neighbor in 2002.

Today's searches mostly centered on open areas near Amber's school and in the area of Mesa Rock Road, near Interstate 15, Escondido police Lt. Craig Carter said.

Roughly 100 seemingly viable tips in the case -- including numerous possible sightings -- have produced no evidence of what has become of the girl, who has no history of running away from home, Carter said.

There also have been no indications of an abduction, according to Carter.

The last confirmed sighting of Amber was on the morning of Feb. 13, as she was walking to school.

The girl's family plans to lead more searches Saturday and Sunday, working out of an operations center in a vacant commercial space at 755 N. Quince St., the lieutenant said.

Authorities asked anyone with information on the missing teenager's possible whereabouts to call 760-743-TIPS.