The election commission in Wisconsin has received a request for a recount of the votes in the state narrowly won by Donald Trump more than two weeks ago.
The request was filed by Green Party presidential candidate Jill Stein.
Dr Stein has also pledged to file vote recounts in Michigan and Pennsylvania.
A win by Hillary Clinton in Wisconsin alone would not overturn Mr Trump’s lead – it provides only 10 votes in the crucial electoral college that gave him victory in the 8 November election.
In a tweet, the Wisconsin Elections Commission said: “The Commission has received the Stein and Del La Fuente recount petitions.”
It added that details would be released shortly.
Meanwhile, Dr Stein tweeted that the recount would begin next week. Friday was the deadline for the request.
Voting rights lawyers who urged candidates to request recounts, John Bonifaz and J Alex Halderman, say the result needs to be closely analysed.
The fact that the results in the three states was different from what polls predicted was “probably not” down to hacking, Mr Halderman said. Concerns over possible Russian interference had been expressed in the run-up to the vote.
“The only way to know whether a cyber-attack changed the result is to closely examine the available physical evidence ,” he wrote.
Pennsylvania’s deadline is Monday, and Michigan’s is Wednesday.
US officials have said there was no evidence of election tampering in the three states where Republican candidate Donald Trump had razor-thin victories over his Democratic rival.
Mr Trump’s camp has made no public comments on the recounts issue.