Federal prosecutors are diving deep into Rudy Giuliani's finances and consulting work in Ukraine as part of a broadening criminal investigation, The Wall Street Journal reported.

The investigation is focused on an alleged conspiracy involving two of Giuliani's associates, Lev Parnas and Igor Fruman, who were arrested last week and are accused of concealing the funneling of hundreds of thousands of dollars of foreign money into US political campaigns.

Prosecutors are reportedly examining Giuliani's role in pushing for the ouster of the US's former ambassador to Ukraine, Marie Yovanovitch, as well as his work for the mayor of Kyiv, Vitali Klitschko.

Giuliani is pivotal in Yovanovitch's abrupt dismissal, and he is also a central figure in President Donald Trump's efforts to pressure the Ukrainian government to dig up dirt on former Vice President Joe Biden and his son, a whistleblower complaint lodged in August alleges.

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Federal prosecutors from the Southern District of New York are sifting through the bank records and foreign business dealings of Rudy Giuliani, the former New York mayor and President Donald Trump's personal lawyer, The Wall Street Journal reported.

The scrutiny comes as part of a broadening criminal investigation into an alleged conspiracy involving two of Giuliani's Ukrainian associates, Lev Parnas and Igor Fruman. The two men were arrested and charged last week with violating campaign finance law connected to hundreds of thousands of dollars in donations to US political campaigns.

The indictment accuses Parnas and Fruman of trying to conceal the source of the payments and funneling foreign money into domestic politics. One of the people they contributed to, prosecutors said, is a former Republican congressman who was part of a lobbying campaign to oust Marie Yovanovitch, who served at the time as the US ambassador to Ukraine.

In addition to examining Giuliani's finances, The Journal reported that federal prosecutors have interviewed witnesses about the former New York mayor since at least August about his potential role in the alleged conspiracy involving Parnas and Fruman.

Read more: FBI officials were 'rattled' and 'blindsided' by Trump's call for Ukraine to manufacture dirt on Joe Biden

Giuliani told The Journal he hadn't done anything wrong and had not been informed of any investigation, adding, "They can look at my Ukraine business all they want."

Giuliani has represented Trump since last spring. He is a key player in efforts to urge the Ukrainian government to dig up dirt on former Vice President Joe Biden and his son, Hunter, related to the latter's work for the Ukrainian natural gas company Burisma Holdings.