Scores of volunteers searched a California lake on Sunday for signs of a missing teenager in what San Diego authorities called an unprecendeted response to a missing persons case.

Chelsea King, a 17-year-old honor student at Poway High School in Poway, Calif., about 25 miles north of San Diego, disappeared Thursday while on an after-school run through Rancho Bernardo Community Park, a family spokeswoman told FoxNews.com.

Authorities said Chelsea, a senior and long-distance runner, was last seen at 3 p.m. Thursday before setting out on a routine several-mile jog along an extensive trail system around Lake Hodges, Fox5SanDiego.com reported.

Police found the teenager's black BMW sedan parked inside the park's parking lot with her cell phone inside, according to the station.

The San Diego County Sheriff’s Department, along with the FBI and law enforcement from Riverside, San Bernardino and Orange counties, launched on all-out search for Chelsea using helicopters, infrared devices and search dogs to scour the park's rugged terain.

On Sunday, dive teams reportedly searched Lake Hodges for any sign of Chelsea, described as 5'5" tall with blonde hair and blue eyes and weighing 115 pounds.

"This is the most horrific nightmare you could ever imagine," family spokeswoman Stephanie Dorian said in an interview Sunday with FoxNews.com.

Dorian said that Chelsea, a peer counselor and french horn player with the San Diego Youth Symphony, "usually didn't run alone."

"She's incredibly intelligent, well-liked and beautiful," Dorian said. "She's adored by everyone."

Dorian said over 2,500 volunteers are assisting law enforcement in the search and that 15,000 missing person fliers have been distributed.

San Diego authorities are reportedly working to determine whether there is any connection between the girl's disappearance and an assault on a woman at the park on Dec. 27.

Click here to read more from Fox5SanDiego.com

Click here to visit FindChelsea.com