CHICAGO (CBS) — A man with a concealed carry permit got involved when he witnessed an armed robbery at a wireless store in southwest suburban Crestwood over the weekend.

WBBM Newsradio’s Mike Krauser reports the man told police he was walking into the AT&T store at 4756 W. Cal Sag Rd. on Saturday, when he saw an armed robbery taking place inside. The man said he saw someone inside with a Mohawk haircut, and armed with a gun.

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He told police he stayed in his car to prevent anyone else from entering the store.

A customer was pistol-whipped during the robbery, and she and two employees got out as a police officer was arriving, after he was flagged down by several witnesses.

The officer saw the robber running out the back, carrying a large duffel bag and a gun. When the suspect spotted the officer, he dropped the bag and ran away. The officer began chasing the suspect, but soon heard two gunshots, and he took cover, not knowing who was shooting.

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It turned out the man with the concealed carry permit had entered the store and fired at the suspect from the back door as the officer arrived.

Crestwood police said the man with the concealed carry permit told investigators he was concerned for the officer’s safety when he saw him go behind the building after the robber went out the back door. No one was injured as a result of the gunfire.

No charges will be filed against the man who fired at the suspect, who later was caught nearby. Demetrius Merrill, 17, of Chicago, was arrested and charged with armed robbery.

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Witnesses said a second armed suspect also was involved in the robbery, but no other arrests have been made. Crestwood police said detectives are following leads.