The robbery happened around 9:10 a.m. at a U.S. Bank location in the 8500 block of West Dempster Street in Niles, police said.
A male suspect, who was wearing a "COVID-type mask and a backpack," entered the bank and demanded money while aiming a handgun at employees, police said.
Investigators said the suspect was able to flee the bank with stolen cash.
No one was injured during the robbery, Niles police said. Schools in the area were placed on lockdown during the police response and investigation.
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The FBI continues to investigate the bank robbery.
No further information about the crime was immediately available.