The IRS performs the offsets and has the following page which discusses why offsets are done and their notification procedures. If you were not notified of an offset by the IRS, you should contact the Financial Management Service at 800–304–3107, Monday through Friday from 7:30AM to 5 PM (Central Time).
https://www.irs.gov/uac/tax-refund-withholdings-and-offsets
