Nearly 400 absentee ballots from Milwaukee, Wis. were not opened by Election Day, a recount paid for by President Trump's team found.

The mistake was due to human error, the city’s election officials announced.

President-elect Joe Biden won Wisconsin by 20,600 votes. Trump had paid for a recount in just Milwaukee and Dane counties, the counties with the most votes for Biden.

The campaign paid $3 million dollars for the partial recount, as mandated by state law, hoping to discover and reject ballots with missing addresses allegedly filled by election officials.

WISCONSIN OFFICIALS: TRUMP OBSERVERS OBSTRUCTING RECOUNT 

The city’s top elections official, Claire Woodall-Vogg, said that the 386 votes would now be added to the final tally, as Wisconsin has not yet certified its election results.

“If there’s one positive to come out of the recount it’s that indeed that every vote is being counted, including these 386,” Woodall-Vogg said.

The unopened ballots were found beneath a pile of ballots that had been counted, she said.  The county board of canvassers voted unanimously to include the ballots as part of the recount.

As of Tuesday morning, recounted ballots, not including the newly discovered ones, showed a net gain of 57 votes for Trump.

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The recount got off to a slow start last week, but as of Tuesday, the work was "very close to being back on schedule" and could be completed as soon as Wednesday, said Brian Rothgery, spokesman for the Milwaukee County Board of Supervisors.

The recount must conclude by Dec. 1, the deadline for Wisconsin to certify its results.

The Trump campaign is widely expected to file a lawsuit challenging tens of thousands of absentee ballots after the recount is complete.

And while the Wisconsin recount continues, Pennsylvania, Michigan and Nevada certified their results in favor of Biden this week.

But Trump’s campaign is appealing a federal judge’s dismissal of his campaign’s effort to block the certification of votes in Pennsylvania.

The Third Circuit Court of Appeals granted the campaign's request for an expedited review of the dismissal, where the judge issued a scathing order shooting down claims of widespread irregularities with mail-in ballots.

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The case was seen as a long shot to stop Biden’s inauguration, but was one of the Trump campaign’s best hopes to affect the election results through the courts, given the state’s 20 electoral votes at stake.

Fox News' Brooke Singman and The Associated Press contributed to this report.