WASHINGTON (AP) — A lawyer who previously advised Michael Cohen, the key government witness in the hush money payment investigation into Donald Trump, has been invited to appear Monday before a Manhattan grand jury that is considering potential charges against the former president, according to a person familiar with the matter.

Robert Costello had advised Cohen before the two had a falling out, and is prepared to offer testimony to the grand jury attacking the credibility of Cohen’s statements, according to the person, who insisted on anonymity to discuss secret legal proceedings.

Costello had contacted a lawyer for Trump saying that he had information that contradicted Cohen’s current statements and that could be exculpatory for Trump, the person said. The lawyer brought it to the attention of the district attorney’s office, which last week subpoenaed Costello’s law firm for records and invited him to provide testimony on Monday afternoon.

The Manhattan district attorney’s office declined to comment on Sunday. Prosecutors have not said when they expect to conclude their work or when or if Trump might be charged. The former president claimed in a post on Saturday that he would be arrested on Tuesday, though people close to him later said that he had been given no updates from prosecutors.

The New York Times first reported on Costello’s expected appearance before the grand jury.

Cohen is a former lawyer and fixer for Trump who has been a key witness in District Attorney Alvin Bragg’s investigation, meeting regularly with prosecutors and appearing before the grand jury earlier this month.

Cohen served prison time after pleading guilty in 2018 to federal charges, including campaign finance violations, for arranging the payouts to porn actor Stormy Daniels and model Karen McDougal to keep them from going public. Trump has denied the affairs.

Costello is a veteran New York lawyer who has represented Trump allies including his adviser Steve Bannon and his personal lawyer Rudy Giuliani. Costello’s connection to Cohen was highlighted in the report of special counsel Robert Mueller, who investigated links between Russia and Trump’s 2016 presidential campaign as well as efforts to obstruct the probe.

According to the Mueller report, Cohen began speaking with Costello in April 2018, days after the FBI raided Cohen’s home, hotel room and office. Amid public speculation about whether Cohen would begin cooperating with investigators, Costello emailed Cohen to reassure him that he had had a “Very Very Positive” conversation with Giuliani and that “you have friends in high places.”

Cohen has waived his attorney-client privilege with Costello, which would allow Costello to describe their conversations, the person said.