CHICAGO (AP) — An Illinois sheriff is warning residents about a sophisticated phone scam involving a person posing as an Internal Revenue Service agent.

Cook County Sheriff Tom Dart says the scam begins with calls made to homes in which an automated voice tells residents that they're the subject of an arrest warrant and provides them a telephone number to call to clear up the matter.

Those calls are answered by the fake IRS agent, who in some instances knows and can recite the last four numbers of callers' social security numbers. The "agent" tells callers that if they don't make a payment the warrants will be passed on to law enforcement.

Dart says genuine IRS agents don't work with law enforcement in that way and they never call without first sending prior notification in the mail.