The violin was made by 18th-century Italian craftsman Nicolo Gagliano and Sabina Nakajima, 24, had borrowed the instrument from a dealer while considering whether to buy it. She left the violin in the trunk of her car, which she parked in a supermarket lot while she went to work elsewhere.
"I thought it was pretty safe in my car," a crestfallen Nakajima told KTVU television. "There's no broken windows and I'm sure I had locked the car."
Police inspector Vince Repetto said authorities are interviewing officials at the towing company as well as seeking other leads.
The violin was insured by its owner, who had loaned it to the dealer so it could be sold, KGO-TV reported.