Among other things, per WSJ:
The subpoenas offer the clearest indication yet that federal prosecutors are examining Mr. Giuliani’s consulting work. Among the entities named in the subpoenas are Giuliani Partners, a security-consulting firm founded by Mr. Giuliani in 2002 that had multiple foreign clients, including a city in Ukraine. The subpoenas also sought information on a company co-founded by Mr. Parnas that paid Mr. Giuliani for business and legal advice.
Mr. Giuliani said in an interview that he hadn’t been contacted by prosecutors and has denied wrongdoing.
Subpoenas described to The Wall Street Journal listed more than a half dozen potential charges under consideration: obstruction of justice, money laundering, conspiracy to defraud the United States, making false statements to the federal government, serving as an agent of a foreign government without registering with the Justice Department, donating funds from foreign nationals, making contributions in the name of another person or allowing someone else to use one’s name to make a contribution, along with mail fraud and wire fraud.