A search warrant has been executed by federal investigators at the Manhattan apartment of former president Donald Trump's personal attorney Rudy Giuliani, in relation to a criminal investigation underway for more than two years, according to US media reports.

Investigators with search warrants combed Giuliani's residence and a separate office, seizing electronic devices, The New York Times reported.

Giuliani has been under scrutiny by Manhattan federal prosecutors since early 2019.

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The ongoing investigation is related to Giuliani's activities in Ukraine on whether he conducted illegal lobbying for Ukrainian officials while he pursued an investigation linked to Trump's primary political rival, President Joe Biden, CNN has reported.

Giuliani spent months trying to help Trump, who was impeached for seeking political help from Ukraine, find dirt on Biden, and his son Hunter Biden, during 2018-2020.

However, Giuliani is yet to be charged and has denied wrongdoing.

Although it is unusual for prosecutors to execute a search warrant on a lawyer, Manhattan federal prosecutors have done so before, most notably in recent years against another former lawyer of Trump, Michael Cohen.

A search warrant for a lawyer like Giuliani would require sign-off by the highest levels of the Department of Justice, likely by Deputy Attorney General Lisa Monaco, according to departmental guidelines.

According to reports, the FBI and the Justice Department's prosecutor in New York had sought search warrants for Giuliani's phones last year, but were denied that while Trump remained in office, until January 20.

Then-Deputy Attorney General Jeffrey Rosen, along with other officials at Justice headquarters and the US attorney's office in Manhattan, decided not to make a final decision.

Under the new attorney general, Merrick Garland, the warrants were evidently approved.

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The New York Times quoted Giuliani's attorney, Robert Costello, as calling the raids Wednesday "legal thuggery."

"Why would you do this to anyone, let alone someone who was the associate attorney general, United States attorney, the mayor of New York City and the personal lawyer to the 45th president of the United States?" he asked.

On January 11, the US Treasury imposed sanctions on four Ukrainians who it said helped Giuliani in the effort to smear Biden ahead of the 2020 presidential election.