Detroit's police chief says a security guard briefly left a checkpoint when a bag containing a gun passed through an X-ray machine, causing a lockdown and evacuation at a government building.

The owner of the bag picked it up and continued walking into the Coleman A. Young Municipal Center on Monday morning before the image of a gun was spotted on a screen.

Police Chief James Craig said Tuesday that the guard shouldn't have left the checkpoint.

The two-tower center is home to Detroit city hall and Wayne County civil courts. It reopened Tuesday.

A man whom police described as "a person of interest" was interviewed, but Craig says he didn't acknowledge having a gun and won't be charged.

Craig says eight guns have been confiscated at the checkpoints in recent months.