UNDATED -- Investigators have uncovered evidence linking a San Ramon teenager killed on a highway onramp in Martinez to a homicide committed hours earlier in nearby unincorporated Contra Costa County, according to Contra Costa County Sheriff's spokesman Jimmy Lee.

At about 12:15 a.m. Saturday the Contra Costa County Sheriff's office received a call from a citizen asking deputies to conduct a welfare check on a man seen lying in a driveway near the intersection of Orange Street and Shell Avenue in an unincorporated area of the county near Martinez, Lee said.

Responding deputies found 44-year-old Robert Downer fatally shot in his driveway, Lee said.

As the sheriff's office launched an investigation into Downer's murder, the California Highway Patrol was investigating the death of 18-year-old Mia Freeland, who was found on Interstate Highway 680, Lee said.

Freeland's body was found on the Marina Vista onramp to the highway shortly after 10:30 p.m. Friday, about two hours before Downer's body was discovered.

Freeland had been in a Nissan Maxima that apparently rolled over an embankment, according to CHP Officer Aaron Quistad.

Lee said it appeared that Freeland, who was found outside the vehicle, could have been involved in the crash before walking along the road where she was killed. It was not immediately clear how she was killed, however the CHP is investigating the case as a possible hit-and-run.

As they were investigating Freeland's death, CHP officers discovered the Maxima was registered to Downer, said CHP Officer Scott Yox. Investigators also found a loaded .22-caliber revolver in the car with one spent casing, Yox said.

"We do believe there is a link between this collision and our shooting," Lee said.

Evidence was sent to the county crime lab for further testing, according to Lee.

Investigators believe Freeland and Downer knew each other, but the nature of their relationship was not immediately clear, Lee said.

Yox said the CHP is continuing to investigate whether Freeland was struck by a hit-and-run driver and urges any witnesses to call the CHP at (925)646-4980.