CHICAGO (CBS) — Residents in some North Side neighborhoods were flummoxed, with mail going missing again and again.

Was it another postal service gaffe? No siree. It turns out that someone had a key they should not have.

As CBS 2’s Jermont Terry reported Thursday night, the frustration from the residents has been overflowing – and not just from packages getting stolen. One property manager said despite police making an arrest, the crook keeps bonding out.

Video shows two guys casually walking into a condo building on Diversey Parkway. They are seen entering easily using a U.S. Postal Service master key, and once inside, they head straight to the mailroom.

“We feel a little violated, you know, that somebody would come and do that,” one condo owner said.

Owners are scared to show their faces and there’s a reason why. If you look closer at the video, one of the guys heading to the mailroom has a knife with him.

“I’m just really relieved nobody ran into them,” the condo owner said.

But like many of the mailroom thefts, people question how the crooks are getting a hold of the keys.

“It’s scary how it was so easy,” the condo owner said. “I mean, if they can do it here, they can do pretty much anywhere.”

It is believed the same man is behind the same crime at a high-rise apartment building in Ravenswood. The property manager there said she had seen the same man on video before.

Two weeks ago, the Ravenswood building’s surveillance system first spotted the thief using a crowbar with a buddy to get into the building and going to the mailroom.

The property managers added security. But then, the man – who even tries blending in with a dog sometimes – returned twice this week with a master postal key and cleared out the mailroom again.

“Those keys don’t just float around,” the property manager said. “They’re kind of difficult to get.”

At least the keys should be difficult to get. Yet this trend has been on the rise across the Chicago area for nearly a year.

But what frustrates the Ravenswood property manager most is that she says police have arrested the suspected thief recently – so how is he back stealing bikes and private information again?

“By the time they’re done writing reports, they’re bonded out and resuming exactly what they’re doing right now,” the property manager said.

The video from both incidents has been turned over to police. But it is up to the individual residents to make a report once they realize something is missing.