While the search warrant does not reflect that Giuliani is guilty of any crime, it shows that investigators have taken the probe to an "aggressive new phase", the Times noted.
"It's a bigger deal to execute a search warrant of an attorney who worked for the former president", she added. The warrants, requiring approval from the top levels of the Justice Department, signify prosecutors believe they have probable cause that Giuliani committed a federal crime - though they don't guarantee charges will materialize.
Giuliani's lawyer, Robert J. Costello, responded to the raid, stating, "What they did today was legal thuggery".
Giuliani's dealings with Ukraine led, in part, to Trump being impeached for the first time in December 2019 on allegations he pressured Ukraine's president to announce the launching of a criminal investigation into Biden and his son. The New York Post reported that "a call to Giuliani's cellphone went straight to voicemail Wednesday afternoon".
The search warrant mentioned an investigation into a possible violation of foreign lobbying rules, and that it meant to obtain communications between Giuliani and certain individuals, Costello told the Wall Street Journal. Even as he was acting as Trump's personal attorney, he was also maintaining a private consulting business that sold services to foreign leaders, including prominent clients in Ukraine, legal experts said.
Federal prosecutors in Manhattan had pushed previous year for a search warrant for records, including some of Mr Giuliani's communications, but officials in the Trump-era Justice Department would not sign off on the request, according to multiple people familiar with the investigation who insisted on anonymity to speak about an ongoing investigation.
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As part of the same investigation, agents on Wednesday also searched the home of Victoria Toensing, a lawyer and Giuliani ally, according to a spokesperson for Toensing.
Hunter Biden's payments by a Ukrainian company at the time his father was vice president have embarrassed the family, but investigators have concluded that no laws were broken.
That consulting work posed no risks when it was confined to foreign soil, but raising anything on behalf of foreign clients with US officials could raise potential violations of the Foreign Agent Registration Act, which requires lobbyists to disclose when they are working on behalf of foreign governments, according to legal experts.
Giuliani's attorney, Bob Costello, reportedly told Fox News that the raid was "totally unnecessary" and was conducted only to give Giuliani "bad publicity" and "make him look like he's some sort of criminal". "I didn't use most of it", Solomon said of Giuliani. Widely admired as "America's mayor" following his response to the September 11, 2001, World Trade Center attack, Giuliani has in more recent years cultivated a more pugnacious and partisan image.
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.
The US Attorney's office in Manhattan and the FBI's NY office declined to comment on Wednesday (Thursday AEST).