The boy managed to aid an arrest during his five-hour shift
Seven Chicago policemen face discipline after a 14-year-old boy impersonated an officer for several hours.
The teenager spent more than five hours acting as an officer after sneaking into the station dressed in a store-bought police costume last month.
Authorities have admitted that he spent two hours behind the wheel of a squad car and went on five assignments.
Supt Jody Weis said that the case exposes troubling lapses in police department policy.
She said the teenager slipped past officers on 24 January, after sneaking into the police station through an unlocked back door.
Despite arriving with no badge or identification, he was taken to be a bona fide officer and handed a radio.
He was then sent out on regular assignments, including an arrest, when the teenager pinned the suspect's arms behind his back, so that another officer could handcuff him, the police said.
The ruse was only noticed several hours later because the boy's uniform lacked a regulation star and he was not carrying a weapon.
He was released from juvenile detention last week after pleading not guilty to impersonating an officer, and is being kept under electronic monitoring at his home.
The seven police personnel taken in by the hoax could face disciplinary action ranging from a reprimand to dismissal.
"They weren't paying attention. They were lax, I'm very upset. This whole incident is very disturbing," said Supt Weis.