Masked thieves who held up a Garda Armored Car Service facility in southwest Santa Rosa made their move as one of the company's armored vehicles pulled into the low, nondescript building late Thursday night, police said Friday.

Authorities said at least two gunmen made their way inside and quickly overtook two employees, who were "disarmed, tied up, and forced to cooperate with the suspects while being held at gunpoint," police Sgt. Steve Fraga said.

The gunmen robbed the company of an undisclosed amount of cash before leaving, closing a steel door behind them as they fled.

No suspects had been arrested by Friday afternoon.

Neither police nor Garda company officials would comment on the amount of money taken. But Garda company officials offered a $100,000 reward for information leading to the arrest of the robbers.

Beyond a broad outline of the 10:45 p.m. heist, police closely safeguarded details of the incident, including whether investigators suspect an inside job.

"It's a very significant loss," Fraga said. "We're reluctant to give details because the only people who are going to know are the people who planned it."

Detectives and Garda staff were taking steps to determine if company staff were involved in the crime, Fraga said.

Detectives also were examining surveillance video for leads.

The robbery came to light when one of the two victims called 911 to report that a robbery had just occurred and they were inside the Northpoint Parkway building, an operational hub used by the international, Montreal-based company.

Both victims were unharmed, but they told dispatchers that the robbers were armed and had tied them up, police said.

It was not clear if the armored vehicle was followed to the site or if it was ambushed. The building is an unremarkable beige stucco building and there are no signs announcing it is an armored car service company.

Also unclear was whether the car itself drove into the building — which has a metal roll-front door that appears large enough to admit a vehicle — or remained parked outside.

Police Lt. Paul Henry would not say if one or both victims had been in the car or how many robbers were seen, though those that were apparently all carried firearms.

Early reports that the victims had been locked inside were unfounded, Henry said.

The building houses a variety of different businesses, though no names are visible from the outside.

Garda's suite is at the rear and appears to be accessed through the rollfront door and another, regular door with an overhead surveillance camera.

Five Garda armored vehicles were parked outside Friday in a parking lot bordered by the building on one side and marshy, Roseland Creek on the other. They were not visible from the street.

Two police field technician vehicles were parked nearby, and someone was working inside a locksmith's van parked among the armored cars.

A uninformed man bearing Garda insignia said he could not comment on the case, and said no one inside would comment.

In addition to the company reward, a $2,500 reward is being offered for information leading to arrests in the case, through the Sonoma County Alliance's "Take Back Our Community" program, police said.

Anyone with information may call police Detective Ryan Corcoran or Sgt. Steve Fraga at Santa Rosa police violent crimes investigation unit, at (707) 543-3590.

(Press Democrat Staff Writer Julie Johnson contributed to this story.)