Rudy Giuliani, President Donald Trump’s personal lawyer, is being investigated by federal prosecutors for possible campaign finance violations and a failure to register as a foreign agent as part of an active investigation into his financial dealings, according to three U.S. officials.

The probe of Giuliani, which one official said could also include possible charges on violating laws against bribing foreign officials or conspiracy, presents a serious threat to Trump’s presidency from a man that former national security adviser John Bolton has called a “hand grenade.”

A second official said Giuliani’s activities raise counterintelligence concerns as well, although there probably wouldn’t be a criminal charge related to that. The officials, who asked for anonymity to discuss a sensitive matter, provided the first indication of the potential charges under investigation.