MONTERA -- Two masked men tied up a U.S. Postal Service driver and left him bound in his delivery truck in this small seaside community in San Mateo County, authorities said.

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The contract driver was tied up for more than three hours before he was found around 10 p.m. Tuesday in Montara, about 15 miles south of San Francisco, postal inspector Jeff Fitch said Wednesday. Workers began searching for the driver after he didn't show up at the San Francisco processing center.

The driver did not appear to be injured, and Fitch said he believed no mail had been taken, Fitch said. Investigators are still trying to determine a motive for the crime.

The postal service was offering a $50,000 reward for information leading to the two suspects.

Robbing a postal employee, contractor or facility is a felony that carries a maximum penalty of up to 25 years.

"One robbery is one too many," Fitch said. "We take these things very seriously. It's a high priority investigation for us."

Tuesday night's incident is the latest crime involving postal workers in the San Francisco Bay area, Fitch said. In November, a letter carrier was shot in the leg when two men attempted to rob him.

In December, a female postal worker was robbed at gunpoint in an apartment complex in El Cerrito. And in January, an armed man walked into an El Cerrito post office and demanded money.