The Justice Department is investigating whether former Interior Secretary Ryan Zinke lied to his agency’s inspector general investigators, a potential criminal violation, The Washington Post reports. Zinke, who left the Trump administration on Wednesday, was facing two inspector general inquiries regarding real-estate deals in his home state of Montana and his involvement in reviewing a proposed casino project by Native American tribes in Connecticut. The investigators came to believe Zinke had lied to them, and they referred the matter to the Justice Department to consider whether any laws were violated, the Post reports.

The Justice Department’s interest is significant, though department officials have reportedly not yet decided whether he should face charges. It is difficult to prove someone has made false statements because it requires investigators to show a person “knowingly and willfully” lied. However, several former Trump advisers have already pleaded guilty to lying to investigators. A spokesman for Zinke said he had not been contacted by the Justice Department. Zinke has not publicly addressed the ethics allegations against him.