The website has been down most of the day and wasn't operational as of 4:30 p.m. EDT.
Mnuchin commented on the outage when asked by reporters after an event in Derry, New Hampshire, where he promoted the Trump tax overhaul along with Ivanka Trump.
Mnuchin described the problem as a "high-volume technical issue" that the IRS is attempting to resolve.
Tax day falls on April 17 this year because April 15 was a Sunday and April 16 was Emancipation Day, a holiday in Washington, D.C.
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