The Wall Street Journal reported that Giuliani Partners is one of the entities named in the subpoenas issued to potential witnesses in a case that appears to involve possible money laundering, obstruction of justice and campaign finance violations, as well as other crimes.
The subpoenas, which went out to people with ties to Giuliani, "suggest that prosecutors are looking closely at the work of Mr. Giuliani himself, according to people familiar with the matter," the Journal reported.
The newspaper also reported the subpoenas seek materials related to two pro-Trump groups, America First Action and America First Policies.
The Manhattan U.S. attorney's office, which is conducting the investigation, already is prosecuting two of Giuliani's associates, Lev Parnas and Igor Fruman, who are charged with campaign finance crimes.
Giuliani, a former New York mayor who previously headed that same Manhattan federal prosecutors' office, has not been criminally charged.
Robert Costello, a lawyer representing Giuliani, told CNBC, "I have yet to find anybody that's actually seen the subpoeonas."
"I would like to see one, then I could make a reasonable judgment of what is going on," Costello said.
Costello, who is a former prosecutor in the Manhattan U.S. attorney's office, said it would be "highly unusual, and, frankly, highly inappropriate," if the subpoenas actually listed more than a half dozen possible criminal charges under consideration, as the Journal reported they do.
Asked if he believes that Giuliani is a target of the investigation by prosecutors, Costello said: "I can't say that he is. I don't know."
"It would be helpful if I saw the subpoena," the lawyer. "I can tell you that he didn't do anything wrong, and he has no fear of any investigation of what he did."
Parnas and Fruman helped Giuliani in an effort to get the government of Ukraine to open investigations into former Vice President Joe Biden and his son Hunter.
Trump's request to Ukraine to conduct such a probe of the elder Biden, who is seeking the Democratic presidential nomination, ignited ongoing impeachment proceedings by the House of Representatives against the president.
CNBC reported Sunday night that Parnas' lawyer has said his client is prepared to testify under oath that aides to Rep. Devin Nunes, the top Republican on the House Intelligence Committee, scrapped a trip to Ukraine this year when they realized it would mean notifying Democratic Chairman Adam Schiff.
The lawyer, Joseph Bondy, declined to comment Monday on the Journal's report.
Earlier Monday, Trump was asked by a reporter about Giuliani, who had recently told Fox News that "I have insurance" if Trump threw "me under the bus."
Giuliani later said on Twitter that the "insurance" he was talking about were "files in my safe" that purportedly detail how Joe Biden's family has been "monetizing" his political posts over the years.
Trump on Monday, referring to Giuliani's comments, said, "Rudy's a great guy, he covered that himself."
"Rudy is the best mayor in the history of New York, strongest mayor, great crime fighter corruption fighter, here and also when he was U.S. attorney," the president said.
"Rudy's a great person and I think that maybe the press isn't treating Rudy well."
— Additonal reporting by CNBC's Christina Wilkie.