LOS ANGELES (AP) — Thieves broke into an unattended Brink’s tractor-trailer parked at a remote Southern California rest stop last week and stole millions of dollars worth of jewelry and gemstones after the vehicle’s two armed guards walked away, authorities said.

The tractor-trailer had been traveling overnight from a jewelry show in the San Francisco Bay Area down to the Los Angeles region for another event. It was not immediately clear whether the suspects followed the vehicle along its 375-mile (603.5-kilometer) journey or stumbled across it at the rest stop in a crime of opportunity, according to Los Angeles Sheriff’s Sgt. Michael Mileski.

The guards reported the heist around 2 a.m. on July 11 after they noticed that the trailer’s lock had been tampered with, Mileski told The Associated Press on Monday.

Investigators are trying to determine how the lock was broken. The FBI is also working on the case.

“If I had that much money in jewelry, I think I would have the most secure lock on it,” Mileski said.

It was not immediately clear how long the guards had left the vehicle unattended at the “Flying J” rest stop and gas station in the unincorporated community of Lebec, about 75 miles (120.7 kilometers) north of downtown Los Angeles.

The merchandise had been loaded onto the truck late on July 10 following an exhibit hosted by the International Gem and Jewelry Show in San Mateo, south of San Francisco, said Brandy Swanson, the group’s director. It was going to an event at the Pasadena Convention Center just northeast of Los Angeles, she said.

While Swanson on Sunday said 18 victims were reporting more than $100 million in losses, Brink’s said the stolen items were worth less than $10 million.

“According to the information the customers provided to us before they shipped their items, the total value of the missing items is less than $10 million,” Brink’s said in a statement. “We are working with law enforcement, and we will fully reimburse our customers for the value of their assets that were stolen, in accordance with the terms of our contract.”

Representatives for Brink’s and the International Gem and Jewelry Show did not immediately respond to requests for comment on Monday afternoon.

Swanson said vendors who travel between jewelry shows typically underinsure their merchandise because they can’t afford to insure it fully.

“That’s where the discrepancy comes in. These are mom-and-pop operators,” Swanson said. “They’re devastated. Some of these people have lost their entire livelihoods.”

Mileski said detectives are currently estimating the theft at $10 million, based on the insurance, but they expect the amount to climb. Investigators are reviewing video footage from local businesses to see if the crime was recorded. The sergeant said multiple suspects are believed to have executed the heist.

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