United States Postal Service officials say thieves broke into four mail trucks over the past two months in San Jose.
The post office is offering a $10,000 reward with information that might help them catch the person or persons responsible for the thefts.
The most recent incident occurred last week in the Willow Glen neighborhood. Officials said someone broke into a locked USPS truck and took off with a mail bag while a carrier was delivering mail.
"They're getting into locked vehicles," US Postal Inspector Jeff Fitch said. "They're getting in and taking some of the mail."
Dozens of residents have received a letter from the post office alerting them their mail was stolen.
"If there's any compromised information with accounts that's concerning," said Lisa Jenkins, a San Jose resident who received the alert from the post office.
Jenkins said the letter was short on details.
Willow Glen residents said any crime in the neighborhood is unexpected.
"I'm very surprised that someone would do something like this," San Jose-resident Lynn Kucera said. "I consider this neighborhood safe and we watch out for each other. So to hear about someone stealing mail from a postal truck is very disturbing."
Some neighbors said they plan to take their important mail directly to the post office to avoid the risk of it being stolen from a truck.
Fitch would not reveal where the other three truck break-ins were in San Jose. Mail theft is a federal crime.