A July 3 video shows a man in a green shirt walking up to the mailboxes at the building on West 34th Street just before 5:30 a.m.
He briefly disappears from the cameras, but when he reappears, he's holding so much mail that he has to put it into a backpack.
The next day, cameras show the man returning in the same shirt.
This time, the video shows him holding something in his hand before fiddling with the mailboxes.
He's able to work on eight boxes at the same time and then leaves with more mail.
"Evidently, he's got a master key, and he was able to open up and steal all the mail out of there," attorney Larry Longer, who owns the building, said.
Longer turned the video over to the Houston Police Department and the U.S. Postal Inspection Service.
HPD confirmed it received the surveillance footage but couldn't say if investigators have viewed it yet. Because the man in the video hasn't been named a suspect, Eyewitness News blurred his face.
In the meantime, Longer is warning his tenants about leaving outgoing mail in the boxes, and he will ensure they get their incoming mail.
"We've asked the carrier to just bring it up to our office for the entire building, and then we spend our time to deliver it to all the tenants ourselves," he said, explaining the lengths he's going.
Longer said he wasn't sure if he or his law firm had any mail stolen, but he's keeping an eye out for anything suspicious.
"I'm monitoring all my bank accounts. I have multiple bank accounts, so there's a lot of checks I have to keep up with and making sure that they all got cashed by the proper people," he said.
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