Updated 1/20/11 – 3:30 p.m.

CHICAGO (CBS) — An angry patron at a Social Security office on the Southwest Side threw a chair at a security guard Thursday morning, causing the guard’s gun to go off.

Chicago police said a customer waiting in line at the Social Security office at 8658 S. Sacramento Av. became upset and got into a heated discussion with an employee Thursday morning.

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During the exchange, the customer opened his jacket and reached into his pocket. A security guard approached the man in an attempt to intervene and, as he did, the customer picked up a chair and threw it at the guard, police said.

The chair hit the security guard, causing the guard’s gun to go off accidentally. No one was hit by the gunshot, police said.

Initially, a source told CBS 2 News that the customer pulled out a gun, but police later said the man didn’t have a gun. It was not clear why the man reached into his coat pocket.

Employees at the office detained the customer until police arrived and took him into custody.

Federal Protective Service spokesman Chris Ortman said the guard was being treated for an injury that was unrelated to his weapon being fired.

Chicago police and federal authorities were investigating. Police said they were looking into whether the man would face state charges or federal charges as a result of the incident, as it occurred at a federal facility.

CBS 2’s Mike Puccinelli and WBBM Newsradio 780’s Steve Miller contributed to this report.

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