Robert Mueller, who is looking into Russia’s alleged meddling in the 2016 United States elections, warned Donald Trump of a subpoena in March if he refused to talk to investigators on the matter, a former lawyer for the president told Reuters on Tuesday. The lawyer, John Dowd, quit Trump’s team two weeks after the incident.

Mueller is investigating whether Trump’s election campaign colluded with Moscow.

In a meeting with Mueller on March 5, Trump’s lawyers had insisted that the president was not obligated to talk with federal investigators, four unidentified officials told The Washington Post. But, the special counsel responded that he would issue him a subpoena to appear before a grand jury if he declined, Dowd told Reuters.

“This isn’t some game,” Dowd claimed to have told the investigators. “You are screwing with the work of the president of the United States.”

After the meeting, Mueller’s team agreed to provide the lawyers more specific information about the topics they wanted to talk about. Trump’s lawyer Jay Sekulow used the details to compile a list of 49 questions that his team believed the president would be asked, three of the four unidentified officials told The Washington Post. The New York Times had published the questions on Monday. Trump had called it “disgraceful” that the questions had been leaked to the media.

Dowd, the lead lawyer in Trump’s team handling the Russia investigation, resigned on March 22, without giving any reasons. He had called the Department of Justice on March 17, seeking to end the inquiry into the alleged ties between Russia and Trump’s 2016 election campaign.

In January, the president had claimed he was looking forward to being questioned. He has often criticised Mueller and denied the allegations of his ties with Russia.