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A bank heist gone awry became a 2 1/2-hour standoff with police on the Northwest Side Monday night, as the would-be robber grabbed a customer and held him hostage before giving up, authorities said.

About 8:30 p.m., the customer emerged from the bank when the alleged attacker turned himself in to officers, said police spokesman Pat Camden.

The customer suffered a minor head wound, but it was unclear whether he had been shot or sustained the injury in some other way. The victim, whose name was not released, was doing business at the TCF Bank at 4930 N. Milwaukee Ave. about 6 p.m.

The alleged thief entered the bank, in a one-story brick building, threatened a teller with a handgun and demanded cash, authorities said.

Police on Monday night did not release the name of the suspect, a 30-year-old man. FBI spokesman Ross Rice said federal authorities are considering charging him Tuesday with attempted bank robbery and, possibly, kidnapping.

With the teller gathering money, an alarm went off, Rice said. As other customers and employees fled and police arrived, the suspect grabbed the hostage and pulled him into the bank’s kitchen, Rice said.

Camden said the gun went off, grazing the man’s head as he was taken hostage. Rice, however, said it was unclear whether the gun fired.

Commuters on the O’Hare “L” line arrived at the Jefferson Park station to find the parking lot turned into a command center, with police massing in riot gear as other officers negotiated with the suspect.

After 2 1/2 hours of phone negotiations, the suspect surrendered his hostage and a .38-caliber revolver, Camden said. Praising the negotiators’ work, Camden called the resolution “a successful operation.”

Attempts to reach managers at Minneapolis-based TCF were unsuccessful.

The victim was treated at Our Lady of the Resurrection Medical Center and released, a nursing supervisor said.

At least one bystander, Robert Ruzecki, 20, waited anxiously for the hostage to be freed. His car had been surrounded by police vehicles while he was at a nearby restaurant.

The bank branch is down the street from the Jefferson Park police station, leading one bystander, Luis Plascencia, to remark of the suspect: “He’s not from this neighborhood or else he’s being stupid.”