The alert comes after three back-to-back robberies on the city's South Side.
Chicago police said the victims were meeting up with someone over an online transaction when they were robbed at gun point.
The crime happened at the following times and locations:
-100-block of W. 103rd Street on July 11 at 7:30 p.m.
-13000-block of S. Greenwood Avenue on July 11 at 5:56 p.m.
-11600-block of S. Ashland Avenue on July 19 at 5:00 p.m.
Police said the suspect is about 18 to 25 years old. He is described to be about 5'8" to 5'6" tall and weigh about 120 to 150 pounds.
The suspect was allegedly armed with a rifle, handgun or knife in each crime.