They say the same people may also be to blame for stealing three collection boxes from spots around Fresno.
Stolen mail is a major concern partly because people who put bill payments in the boxes could end up with late fees and even higher interest rates.
And of course there is plenty of other important mail people need to have delivered. So investigators are now asking for the public's help in stopping these brazen burglaries.
These imprints now mark the spot where a big, blue, collection box for stamped mail stood before someone stole it over the holiday weekend.
"I'm shocked. I think its terrible people resort to this type of thing," General Owens said.
Officials from the Los Banos Post Office say the thieves must have unbolted the box and loaded it into a truck one night between July 3 and the morning of July 6. The postal inspector says it's safe to assume a fair amount of mail was in the box at the time. Kim McAdam used it every day.
"OK, now I'm going to have to go think about what I dropped off, especially for work. Man, people, what this world is coming to, I don't understand," Kim McAdam said.
This post office previously had problems with people stealing mail from boxes in the adjacent alley, so they moved all of the boxes to the front about two years ago. But Joe Trujillo says he was still skeptical.
"I don't trust mailboxes. I usually deposit it into the post office," Trujillo said.
The postal inspector says that's a good idea considering the valley has had four collection boxes stolen in the past month and a half.
"Most of these theft occur at night, so if it's after the last collection time on the box you probably want to take it inside or wait until the next day because even if you deposit it, it's not going anywhere til the next day."
The postal service has just finished upgrading all of the boxes around Fresno to make them much harder to steal. But with limited money and resources, it's unclear when other cities will also get the extra security.
If you think you may have put mail in a stolen box, you can fill out a theft form at your local post office or online at usps.com
Anyone with information about stolen boxes is being asked to call the postal inspector at 559-497-7596.
