Gwinnett County police were working Friday to determine who stole 25 navigation systems from vehicles at a dealership near the Mall of Georgia.

Damage to the vehicles is likely between $125,000 and $150,000, Mark Frost, vice president of operations for Jim Ellis, said Friday afternoon. Frost said the service manager at the Jim Ellis Buick GMC dealership, near Buford, called him early Friday with the news.

Frost said windows on new and used vehicles were smashed out, allowing someone to crawl in and remove the GPS devices with a tool, rather than ripping them out of the dashboard.

"We're self-insured -- most dealerships are -- so we have to pay for this," Frost told the AJC. "That's a big loss."

With a new facility under construction, surveillance cameras were not installed on the lot, which is rented, Frost said. But he still has one nagging question.

"What was our security guard doing?" Frost said.

An overnight security guard works at the lot, but Frost said it's possible he fell asleep. Detectives were interviewing the guard Friday morning and checking each vehicle for fingerprints, Cpl. Jake Smith, Gwinnett police spokesman, told the AJC.

Frost is offering a $5,000 reward for information leading to the arrest and conviction of those responsible for the thefts.