The FBI has carried out searches at the home and office of Rudy Giuliani, who was Donald Trump's personal lawyer.
The raid is part of a probe into Mr Giuliani's dealings with Ukraine. His lawyer says he denies breaking the law.
Before the 2020 presidential election Mr Giuliani led an effort to find incriminating information about Democratic candidate Joe Biden and his son Hunter in Ukraine.
Mr Biden won the election and he and his son have denied any wrongdoing.
Mr Trump was impeached in 2019 over allegations he sought help from Ukraine to boost his chances of re-election.
What do we know about the raid?
Searches took place on Wednesday at the former New York City mayor's Upper East Side apartment, as well as his Park Avenue office.
Several electronic devices belonging to the 76-year-old were seized, according to the New York Times.
The warrants included an allegation that Giuliani failed to register as a foreign agent. The Foreign Agents Registration Act requires people to notify the State Department if they are acting as a foreign agent on behalf of another nation.
Mr Giuliani's lawyer, Robert Costello, called the searches "legal thuggery".
"This is totally unnecessary," Mr Costello told Fox News, adding that the search was conducted to "make him look like he's some sort of criminal".
He added that the raids stem from an "alleged incident of failure to register as a foreign agent".
Mr Giuliani has previously called the investigation "pure political persecution".
What's the background?
Mr Giuliani played a central role in the effort to pressure Ukraine to investigate Mr Biden and his son Hunter, who served on the board of a Ukrainian energy firm. He was also instrumental in the ousting of the former US ambassador to Ukraine, witnesses at Mr Trump's impeachment trial testified.
Hunter Biden is facing an unrelated criminal tax probe from the US Justice Department.
Wednesday's search warrant does not imply criminality on the part of Mr Giuliani, but it signals a ramping up of the case into the former mayor.
Investigators have released few details so far about the case and it remains unknown whether the Republican will eventually face any charges.
The search warrant was signed by a federal judge. Warrants against lawyers such as Mr Giuliani require the authorisation of senior Justice Department officials, the Associated Press reports.
Mr Giuliani began representing the former president in April 2018 when Mr Trump was being investigated for his involvement with Russian interference in the 2016 presidential election.
After Mr Trump's election loss in November, Mr Giuliani toured the country speaking to Republican state legislators in an effort to get state lawmakers to overturn the result in favour of Mr Trump.
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