It happened on Wednesday, August 7, around 3:10 p.m. inside 585 6th Avenue in Park Slope.
Police say the 32-year-old employee of the building noticed three men inside the building's lobby cutting open mailed packages and removing the contents.
The employee confronted the men, who attacked him.
They then ran through the lobby onto 6th Avenue with various clothing items worth $450.
Fortunately, the building employee is expected to be OK.
Anyone with information in regard to the identity of the males is asked to call the NYPD's Crime Stoppers Hotline at 1-800-577-TIPS (8477) or for Spanish, 1-888-57-PISTA (74782).
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