(CNN) For months, investigators looking into Rudy Giuliani's business dealings in Ukraine have dug into everything from possible financial entanglements with alleged corrupt Ukrainian figures to counterintelligence concerns raised by some of those business ties, according to people briefed on the matter.
The counterintelligence part of the investigation indicates that FBI and criminal prosecutors in Manhattan are looking at a broader set of issues related to Giuliani, President Donald Trump's personal attorney, than has been previously reported.
Kenneth McCallion, a New York attorney, says that investigators first approached him earlier this year to ask about Giuliani's ties to Lev Parnas and Igor Fruman, two Giuliani associates indicted last week on campaign-finance related charges.
McCallion says FBI counterintelligence agents in February or March asked questions about some of Giuliani's Ukrainian business dealings.
The counterintelligence probe hinges in part on whether a foreign influence operation was trying to take advantage of Giuliani's business ties in Ukraine and with wealthy foreigners to make inroads with the White House, according to one person briefed on the matter.
"I was just asked whether I or any of my clients knew of any dealings that these two guys had with Giuliani," McCallion said. "They were on the radar with regard to possible counterintelligence issues."
The indictment announced last week centers on ties between the Giuliani associates and foreigners, including a Russian national with whom they did business. The indictment doesn't mention Giuliani.
Giuliani has said he is unaware of any criminal investigation into his business dealings.
McCallion, who first spoke to USA Today, has represented Ukrainian clients, including the former Ukraine Prime Minister Yulia Tymoshenko.
Some of the FBI agents and prosecutors handling the case come from the same public corruption unit that targeted Michael Cohen, the President's former personal lawyer, and it's clear the investigation could result in more charges.
The investigators in the Southern District of New York appear to have largely operated separately from what Trump's appointees at the Justice Department headquarters in Washington, DC, have pursued in recent months, and the investigation dates back far longer than what's been previously reported.
Justice Department officials last month said criminal division prosecutors in Washington had examined Trump's July call with the Ukrainian president only for the narrow issue of potential campaign finance violations and determined there was no crime. But the issue was far from closed.
At the same time, federal prosecutors in New York were aggressively pursuing broader issues related to the Ukraine matters.
The first indication of the broader investigation emerged last week when Manhattan-based investigators pursuing the charges brought last week against four Giuliani associates, including Parnas and Fruman, who were indicted on campaign finance-related offenses.
Nicolas Roos, the top prosecutor from Manhattan who worked on Cohen's prosecution for tax evasion and campaign finance violations, took the lead in Virginia federal court last week when Parnas and Fruman made their initial court appearance.
When the Giuliani associates were first charged, the Justice Department's public integrity team based in Washington were consulted about the charges but were not involved further in the case, according to a person familiar with the matter.
Prosecutors are expected to bring additional charges against at least some of the men, people briefed on the matter say. A grand jury has already issued at least one subpoena, to former Texas Republican Rep. Pete Sessions, another indication that further charges could be in the works. Rules that govern grand juries say that prosecutors can't continue to subpoena witnesses to the secret proceedings unless they are pursuing additional targets or charges.
Sessions wrote a letter to Secretary of State Mike Pompeo to raise concerns about the then-US ambassador to Ukraine, who was later removed by Trump following complaints by Giuliani and others.
The politically charged investigation is broadly examining the financial sources for the work that brought Giuliani to Ukraine, where he worked with Parnas and Fruman and where he previously lobbied the Ukraine government on behalf of a small town in the eastern part of the country. Parnas and Fruman were involved this year in helping find information that Giuliani sought to show alleged corruption by former Vice President Joe Biden and his family. No allegations against the Bidens have been corroborated.
Giuliani was not discussed in the allegations that Parnas and Fruman faced. Instead, the pair and two others, David Correia and Andrey Kukushkin are alleged to have funneled a Russian businessman's money into US campaigns and of a conspiracy to skirt campaign finance requirements as they pushed Ukrainian political interests in the US. They have not yet entered a plea, are being held in jail until they can each post $1 million bond, and are set to appear in court on Thursday afternoon.