The bank robbery happened before 3:30 p.m. at the BMO Harris Bank located at 500 W Half Day Rd. in Buffalo Grove, the FBI said.
The suspect used a note to demand money and implied that he had a handgun, the FBI said.
The FBI has released surveillance photos of the suspect. He is a white man in his 30's, who was wearing a long-sleeved black jacket, a baseball cap, dark pants and white sneakers, and he had facial hair and acne scars on his face.
The suspect remains at large.
No further information was immediately available.