SAN DIEGO -- Somebody has been stealing mail from letter carrier trucks in San Diego, and a large reward has been offered for the thief's arrest.

Postal officials said that it's very rare that a delivery truck gets broken into in San Diego -- except lately. There have been three such incidents in the last three weeks.

Postal inspectors don't want to say exactly where the thefts occurred, but they did say that two of the incidents happened in ZIP code 92102, out of the Golden Hill office. The third incident took place in ZIP 92113, out of the Logan Avenue station. In each case, somebody went up to a parked truck, broke a window and grabbed material out of the truck while the letter carrier was down the street delivering mail.

The thefts are federal crimes, said Postal inspector Hilary A. Smith.

"Each piece of mail stolen carries a maximum five-year sentence and a $250,000 fine," Smith said. "So we take this very seriously."

It is virtually impossible to know how much mail was stolen, considering how many hundreds of thousands of letters are processed and delivered every day in San Diego. Carriers generally have a pretty fair idea of what they're delivering and to whom on any given day.

"Postal inspectors, in turn, send out a personalized letter to everybody in that particular area who has been affected," said Smith.

The letter from the Postal Service says that the person who took the mail may have done so to get information on bank accounts, credit cards, pin numbers or checks.

"We're offering a $10,000 reward for any information leading to the arrest and conviction of individual or individuals who broke into the vehicles and took mail," Smith said.

Anybody with information is being urged to call Postal inspectors through the Postal Service, or to call their local police.