A massive search is under way for a 16-year-old Stephenville girl whose car was found abandoned about 10 p.m. Monday at a roadside park on US Highway 377.

The missing teen has been identified as Alyssa Williams, a freshman at Stephenville High School.

Erath County Sheriff Tommy Bryant said a deputy discovered Williams' car while on routine patrol. Bryant said the car's passenger side door was open, the keys were still in the ignition and Williams' purse and cell phone were inside the car when it was found.

Chief Investigator Donnie Hensley is leading the investigation with the assistance of Texas Rangers, Department of Public Safety, U.S. Marshals and local law enforcement agencies. Hensley said the case is being treated as a "missing person's case with suspicious circumstances."

"At this point, we have a missing 16-year-old female and we are pulling all of our resources together to find her," Hensley said. "We don't know if we are dealing with an abduction or a runaway."

Anyone with information about Williams' disappearance should call the sheriff's office at 254-965-3338.