The businessman, Lev Parnas, is a close associate of Giuliani and was involved in his effort to investigate Trump’s political rival, former Vice President Joe Biden, who is a leading contender for the 2020 Democratic Party nomination.

Giuliani said Parnas’ company, Boca Raton-based Fraud Guarantee, whose website says it aims to help clients “reduce and mitigate fraud”, engaged Giuliani Partners, a management and security consulting firm, around August 2018. Giuliani said he was hired to consult on Fraud Guarantee’s technologies and provide legal advice on regulatory issues.

Federal prosecutors are “examining Giuliani’s interactions” with Parnas and another Giuliani associate, Igor Fruman, who was also indicted on campaign finance charges, a law enforcement source told Reuters on Sunday.

The New York Times reported last week that Parnas had told associates he paid Giuliani hundreds of thousands of dollars for what Giuliani said was business and legal advice. Giuliani said for the first time on Monday that the total amount was $500,000.