Break-ins at about 100 mailboxes in the Fresno County foothills, 14 miles east of Auberry, were reported Thursday.
Mail was taken from six cluster box units, each containing 16 individual boxes and a large parcel box, near the Tollhouse and Peterson roads intersection.
A fire station is nearby, but Karen Nemeth, a clerk at the Auberry post office, said breaking into the cluster boxes can be done without attracting much attention.
"If you know what you're doing, you can do it pretty quietly," she said.
New replacement mailboxes will have more enhanced security measures, she said.
Bill McPike, who gets his mail at the Mountain Rest site, said the back plate of each of the six cluster boxes was pried open and locks were removed.
"There is no way they can lock and the backs are all bent out of shape," he said.
Some residents don't live in the area year-round and mail that is weeks old may have been stolen, McPike said.
Visit fresnobee.com throughout the day for breaking news and updates.