A lawyer for the ex-husband of the woman who had an affair with Missouri Gov. Eric Greitens says prosecutors have convened a grand jury in their investigation into whether Greitens blackmailed the woman.

Al Watkins, the lawyer, announced Monday that his client was served with a subpoena to provide testimony to the grand jury.

The news may represent an escalation of the probe, which was launched last month.

“The issuance of a Grand Jury subpoena conclusively indicates that the Circuit Attorney’s Office has elevated its undertakings to include Grand Jury proceedings, for which subpoena power and other discovery tools are available,” Watkins said in a statement provided to TPM. “The power of subpoena is an invaluable tool to garner evidence and compel testimony which far transcends that which is accorded investigative personnel not otherwise armed with a Grand Jury.”

Greitens, a Republican, has publicly apologized for cheating on his wife and vowed to remain in office through a scandal that has prompted calls for his resignation from members of his own party. He has vehemently denied allegations that he slapped and once took a compromising photograph of the woman that he threatened to release it if she went public about their relationship.

The blackmail claim was made by the woman in a conversation with her then-husband, which he recorded without her knowledge and provided to a local news station.

St. Louis Circuit Attorney Kim Gardner launched the investigation into what she called the “very troubling” allegations against Greitens shortly after they were first surfaced in mid-January.

Watkins has told TPM he began “receiving ongoing consistent contact” about Greitens from an FBI special agent in October 2016.

According to CNN, the FBI recently opened an inquiry into the governor. It’s unclear if the outreach to Watkins is related to the inquiry that two federal officials confirmed to CNN.

The bureau’s St. Louis office has declined to comment on the existence of any investigation related to Greitens.