WASHINGTON — The Interior Department’s internal watchdog has referred an investigation of Secretary Ryan Zinke to the Justice Department, signaling a potential escalation amid a series of inquiries into Zinke’s conduct.

Two people familiar with the investigation said Interior’s office of inspector general referred a case involving Zinke to Justice. It is not clear what the investigation is focused on. They asked not to be identified because they were not authorized to speak publicly on the case.

Zinke faces a number of probes, including one centered on a Montana land deal involving a foundation he created and the chairman of Halliburton, a prominent energy services company that does significant business with Interior.

Investigators also are reviewing Zinke’s decision to block two tribes from opening a casino in Connecticut and his redrawing of boundaries to shrink a Utah national monument.

Zinke has denied wrongdoing, and his lawyer said Zinke has not been notified of any Justice Department investigation or inspector general referral.

“It is disappointing that unsubstantiated and anonymous sources have described an IG office referral to members of the media, as this violates DOJ and IG policy direction. The secretary has done nothing wrong,” lawyer Steve Ryan said in a statement.

Nancy DiPaolo, a spokeswoman for Deputy Inspector General Mary Kendall, said she cannot comment on ongoing investigations.

Spokeswoman Nicole Navas Oxman said Justice “generally does not confirm, deny or otherwise comment on the existence or non-existence of an investigation.”

The referral was first reported by The Washington Post.

Arizona Rep. Raul Grijalva, the top Democrat on the House Natural Resources Committee, said if Democrats win a House majority next week, Zinke will be called to testify “on why his conduct in office merited referral to the Justice Department, whether that referral was related to the recent attempted firing of his inspector general, and his many other failures and scandals.”

Grijalva was referring to a short-lived bid by the Trump administration to reassign a political appointee at the Department of Housing and Urban Development to replace Kendall as acting head of the inspector general’s office.