F.B.I. Searches Giuliani’s Home and Office, Seizing Phones and Computers

Press officers for the F.B.I. and the U.S. attorney’s office declined to comment.

The investigation of Mr. Giuliani grew out of a case against two Soviet-born men who aided his mission in Ukraine to unearth damaging information about Mr. Biden and his son Hunter, who was on the board of an energy company there. Prosecutors charged the men, Lev Parnas and Igor Fruman, and two others with unrelated crimes in 2019, and a trial is scheduled for October.

Prosecutors have examined, among other things, Mr. Giuliani’s potential business dealings in Ukraine and his role in pushing the Trump administration to oust the American ambassador to the country, a subject of testimony at Mr. Trump’s first impeachment trial.

As he was pressuring Ukrainian officials to investigate the Bidens, Mr. Giuliani became fixated on removing the ambassador, Marie L. Yovanovitch, whom he saw as an obstacle to his efforts. At the urging of Mr. Giuliani and other Republicans, Mr. Trump ultimately ousted Ms. Yovanovitch.

Prosecutors have explored whether Mr. Giuliani was working not only for Mr. Trump at the time, but also for Ukrainian officials or businesses who wanted the ambassador to be dismissed for their own reasons, according to people briefed on the matter.

It is a federal crime to try to influence or lobby the United States government at the request or direction of a foreign official without disclosing it to the Justice Department. At least one of the search warrants for Mr. Giuliani’s devices explicitly stated that the possible crimes under investigation included violations of that law, the Foreign Agents Registration Act, according to one person with knowledge of the matter.

Prosecutors have scrutinized Mr. Giuliani’s dealings with Yuriy Lutsenko, one of the officials who helped Mr. Giuliani and his associates in their efforts to tar Mr. Biden while also urging them to work to get the ambassador removed.

Among other things, prosecutors have examined discussions that Mr. Giuliani had about taking on hundreds of thousands of dollars in apparently unrelated consulting business from Mr. Lutsenko, which resulted in a draft retainer agreement that was never executed.