CHICAGO, Ill. (WTVO) — Police said approximately 18 people broke into a U.S. Post Office in Chicago early Monday morning.

According to WLS, the crime happened around 12:35 a.m. in the Thompson Center, at 100 W. Randolph Street in the Loop. The suspects broke the front door and entered the building, police said.

Video of the break-in was shared by the U.S. Postal Inspection Service.

Authorities did not reveal what was taken, although NBC 5 reports that thieves have been targeting postal carriers in an effort to take the master keys that open group mailboxes in lobbies of apartment buildings.

The Chicago Sun-Times has reported that thieves have then used those keys to steal mail containing checks, then washing the ink, re-writing them and then cashing them for thousands of dollars.

A spokesperson for the U.S. Postal Service said, “The US Postal Inspectors and Chicago Police Department are investigating the property damage incident at the Post Office located at the Thompson Center. We are gathering information and will attempt to release information as warranted at a later date.”

Postal inspectors are offering a reward of up to $10,000 for information leading to arrests.