Authorities are hoping the public can help identify a man suspected of robbing five customers in separate incidents at a Lowell bank ATM since January, Lowell police said. The suspect has surprised customers using the ATM at the Washington Street Savings Bank at the intersection of Middlesex and Central streets, said Deputy Superintendent Arthur Ryan of the Lowell police. In the last two robberies, including the most recent on Sunday, the suspect has brandished a knife, though none of the victims were injured, police said. The suspect is described as 20 to 30 years old, 5 feet 6 inches tall, with a medium build.Lauren Dezenski can be reached at email@example.com. Follow her on Twitter @LaurenDezenski.