By Marissa Parra

CHICAGO (CBS) — A CBS 2 investigation.

Thieves in Chicago after private mail. There are pictures of two of them at work. CBS 2 learned how they’re getting through locked doors.

CBS 2’s Marissa Parra explains why the postal service is so concerned about these thefts.

Disturbing video inside a South Loop apartment shows an unnerving trend: Thieves in Chicago with a key that has access to an unknown number of apartment buildings and all the mail inside them.

It’s why postal carrier Lavone Merritt keeps her guard up and her hood down.

“I mean you’re walking down the street to keep the wind out your face, but you still have to be able to see,” Merritt said.

She knows how many eyes are on one small key.

“It’s always connected to me. It’s always hooked to me. I don’t let it out of my sight,” Merritt said.

Inside a USPS cart, little pieces of paper with big secrets. Bank statements, love letters, Social Security and medical information. Things you don’t want strangers to look through.

But that’s exactly what’s happening in the Chicago area. Surveillance video from days ago in the South Loop showed two guys with their own key. Using it not just to get into an apartment building, but also opening up the entire mailbox unit inside and scavenging through the private lives of the people who live there.

“That feels like such a violation,” said resident Kathleen Cunningham. “It could lead to identity theft. It could lead to checks being stolen. So I think it’s really disturbing that someone has the key to get into my mailbox.”

It’s not the first offense. Key thieves have struck multiple parts of the city.

A postal inspector for the Chicago division said they’re “aware” and “investigating” the incidents, working closely with CPD and local aldermen. She added that they’ve “seen an increase” in the number of postal carriers “confronted with force” to hand over their key on their routes.

No one knows for sure how many keys are out there, if it’s just one, or if there are copies or how the key thieves are getting them. And it’s those lingering questions that Merritt said makes it hard to feel safe on the job.

“This is our food on the table. We don’t want to be attacked,” Merritt said.

The solution isn’t as simple as making a new key. That process, which the postal inspector referred to as ‘re-keying,’ takes considerable time and money. Something she said she’s never seen in her 25 years here.

But if the problem persists, a recommendation for the city to go through that time consuming and expensive process isn’t out of the question.