A lawyer for Giuliani, 76, confirmed that search warrants had been executed, and that authorities had seized cell phones and computers. The warrants included an allegation that Giuliani failed to register as a foreign agent, a violation of lobbying laws.

Prior to the 2020 presidential election, Giuliani led an effort to dig up dirt on then-Democratic contender Joe Biden and his son Hunter in Ukraine.

Biden defeated the Republican Trump and is now president. He and his son have denied wrongdoing.

In a statement, Giuliani's lawyer Bob Costello defended his client and suggested the investigation was politically motivated.

"Mayor Giuliani has not only denied this allegation, but offered twice in the past two years through his attorney Bob Costello to demonstrate that it is entirely untrue," the statement said. "Twice the offer was rejected."

Costello said the electronics seized were "replete" with information protected by attorney-client privilege.

Costello later told Reuters investigators also took laptops, plus a desk top computer that belonged to Giuliani's former wife from his apartment, and that they searched his Park Avenue office and took the work computer of Jo Ann Zafonte, Giuliani's longtime assistant.

Zafonte is one of three Giuliani employees who have received subpoenas to appear before a grand jury in Manhattan, Costello said. She is scheduled to appear in late May. Zafonte did not immediately return a request for comment.

A spokesman for U.S. Attorney Audrey Strauss in Manhattan declined to comment.

Federal agents also executed a search warrant on Wednesday at the home of Victoria Toensing and Joseph diGenova, who are spouses, Giuliani associates and former prosecutors, and seized Toensing's cellphone.

Toensing and diGenova have also represented Dmytro Firtash, a Ukrainian oligarch who has been indicted in the United States on bribery and racketeering charges and has fought extradition from Vienna.

Federal prosecutors have been looking at Giuliani for nearly 1-1/2 years.

In November 2019, they sought records of payments to Giuliani as part of an active criminal investigation, according to a grand jury subpoena seen by Reuters.

Prosecutors were investigating money laundering, wire fraud, campaign finance violations, making false statements, obstruction of justice and violations of the federal Foreign Agents Registration Act (FARA), the subpoena said. read more

FARA requires "certain agents of foreign principals" to periodically disclose their relationships with them, including financial transactions.

'SEISMIC MOMENT'

While the search warrant does not mean Giuliani committed a crime, it signals that investigators had reason to believe criminal conduct had occurred, and convinced a judge that a search might uncover evidence of a crime.

"This is a seismic moment in the investigation," said Jessica Levinson, a professor at Loyola Law School in Los Angeles.

"It's a big deal to execute a search warrant concerning an attorney because of issues of attorney-client privilege," she added. "It's a bigger deal to execute a search warrant of an attorney who worked for the former president."

Giuliani gained renown in the 1980s as Manhattan's top federal prosecutor, taking on leaders of top Mafia families and Wall Street's "junk bond king" Michael Milken.

He later won national fame as "America's Mayor" for helping New York City recover from the Sept. 11, 2001, attacks, and was named Time magazine's "Person of the Year."

But Giuliani's image was stained by his dealings with Ukraine, and more recently by his promotion of baseless claims of fraud to overturn last year's election results and give Trump a second White House term.

COMPLEX ROLE

Giuliani began representing Trump, a fellow Republican and longtime fellow New Yorker, in April 2018 in connection with then-Special Counsel Robert Mueller's investigation into Russian interference in the 2016 presidential election.

In Washington, Giuliani's role was complex, with the former mayor frequently proclaiming himself a business consultant and lawyer in the private sector even as he enjoyed extraordinary access to the halls of power.

While Giuliani has yet to be charged with a crime, two former associates, Lev Parnas and Igor Fruman, have been charged with campaign finance violations and other crimes.

Parnas and Fruman helped Giuliani investigate the Bidens, and undertake what prosecutors called an effort to remove then-U.S. Ambassador to Ukraine Marie Yovanovitch.

Hunter Biden had sat on the board of Burisma, a Ukrainian natural gas company.