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Airlines have reported more than 500 cases involving unruly passengers since late December, and most started with passengers who refused to wear a face mask.

That’s according to the Federal Aviation Administration, which said Wednesday it is reviewing more than 450 cases and has started enforcement action in about 20. The FAA is extending a “zero-tolerance” policy against unruly people on planes.

The policy was due to expire at the end of this month, but the FAA says it’ll stay in place as long as passengers are required to wear face masks.