Surveillance cameras and witnesses put the car near Sierra's Morgan Hill home on March 16 and also near where her cell phone and purse were later found.
"It is more than one source and these sources are a combination of residential cameras, business cameras, as well as bus cameras in the area during the time that Sierra disappeared. We just can't give a specific number because it is still an open investigation," said Santa Clara County Sheriff's Dept Sgt. Jose Cardoza.
Sierra's family told investigators they do not recognize the car.
Marlene LaMar, Sierra's mother, says the identification of the car is just the kind of solid lead they have been waiting for and one in which the public can help.
"I know everybody out there is going to do everything possible to report the sighting of this car," Marlene LaMar said.
The car is described as a red, mid-1990s model four-door Volkswagen Jetta with a black hood. Sheriff's investigators say the black hood appears to be a modified feature on the car.
Sierra's father says that detail in itself could prove to be another source of information.
"Anyone that might be seeing this, if they own a body shop and remember doing something like that on a similar car, again, that's useful information," Steve LaMar said.
Sierra was last seen at home getting ready for school. Detectives do not believe she ever made it to the bus stop. Investigators say it's not clear from surveillance video who is in the car.
But Sierra's mom is convinced her daughter is still alive and has a message for anyone involved in her disappearance.
"I am not looking for trouble or a conviction. I just want -- we all want her safe return. We all want this nightmare to end, and that's my plea out there," Marlene LaMar said.
Detectives are not revealing if they have the license plate number of the car they are searching for. If they do have it, they are not releasing that information just yet.
Sheriff's investigators also revealed Monday that they have received lab analysis on two evidentiary items; a box marked handcuffs and two used condoms are not related to the case.
The sheriff's office is asking anyone with information to call investigators at (408) 808-4500 or the anonymous tip line at (408) 808-4431.