Police need the public’s help finding a group of men who they say assaulted a young man. 

It happened inside a Westchester Avenue building around 9 p.m. Sunday.

Police say surveillance video shows three men gathering in the stairwell of the building, where they met the victim to exchange property.

According to police, one suspect pulled a knife and the group then assaulted the victim.

The suspects stole the victim's backpack and cellphone.

Police believe all three suspects are in their late teens. 

Anyone with information is urged to call Crime Stoppers at 1-800-577-TIPS.