The Interior Department’s Office of Inspector General has referred one of its probes into the conduct of Secretary Ryan Zinke to the Justice Department for further investigation, according to two individuals familiar with the matter.

Deputy Inspector General Mary L. Kendall, who is serving as acting inspector general, is conducting at least three probes that involve Zinke. These include his involvement in a Montana land deal and the decision not to grant two tribes approval to operate a casino in Connecticut. The individuals, who spoke on the condition of anonymity because they were not authorized to talk publicly, did not specify which inquiry had been referred to the Justice Department.

A spokeswoman for the inspector general’s office said Tuesday, " I cannot comment on any investigations.” The Justice Department declined to comment, and Interior did not immediately respond to a request for one.

A referral to the Justice Department means prosecutors will explore whether a criminal investigation is warranted. While an agency’s inspector general regularly issues reports on the findings of its inquiries, it refers cases to the Justice Department only when it has determined that there could be criminal violations.

A senior White House official, speaking on the condition of anonymity because of the sensitive nature of the matter, said the White House understands that the investigation is looking into whether the secretary “used his office to help himself.”

Since taking office in March 2017, Zinke has come under scrutiny for his travel practices and other aspects of his job performance, though his aides have dismissed such allegations as politically motivated. This month, the inspector general’s office released a report finding that Zinke’s decision to allow his wife to travel with him sparked concerns among the department’s ethics officials. Zinke is one of multiple Cabinet members who may leave after the midterm elections, according to administration officials.

Zinke is looking for a political nominee who could replace Kendall, a career official who has served in an acting capacity since 2009. Two weeks ago, Housing and Urban Development Secretary Ben Carson sent an email to his staff announcing that one of his top aides, Suzanne Israel Tufts, would be taking over as Interior’s acting inspector general. But Zinke’s aides disavowed this idea several days later, describing Carson’s email as “100 percent false.”

Interior’s inspector general has continued to look into Zinke’s conduct even as the debate over a replacement for Kendall has played out in public.

One of the allegations under investigation regards the secretary’s role in a Montana land development deal backed by David J. Lesar, chairman of the oil services firm Halliburton, which Politico first reported in June. The business and retail park, known as 95 Karrow, is slated to include several businesses and would be near multiple parcels of land owned by Zinke and his wife. The deal involves land owned by a foundation now headed by Zinke’s wife, Lola, which the secretary used to run before joining the Trump administration.

While Zinke has stepped down as president of the Great Northern Veterans Peace Park Foundation, records obtained under the Freedom of Information Act show that he has met with Lesar, his son and the lead project developer, Casey Malmquist, since taking office. He also corresponded with Malmquist about the project’s design, the records reveal.

Halliburton’s operations are directly affected by many of Interior’s policies, including rules on how oil and gas drilling must be conducted, and which public lands and federal waters are open to energy exploration and development.

Separately, the inspector general’s office is examining Zinke’s involvement in his department’s refusal to sign off on a proposed casino deal in Connecticut involving the Mashantucket Pequot and Mohegan tribes. Interior career staffers had recommended approving the tribes' application to jointly run the casino, which would have competed with an MGM Resorts International casino across the border, in Massachusetts. But MGM and two Nevada senators lobbied against the permit, and in the end, Zinke did not grant it.

Interior’s inspector general has subpoenaed documents from MGM, according to two individuals briefed on the inquiry who spoke on the condition of anonymity because the probe is still underway.

Asked about the subpoena this month, MGM Resorts International spokeswoman Debra DeShong said in an email, “We are not in a position to comment in light of the ongoing litigation filed by the Mashantucket Pequot Tribe and State of Connecticut.”

Zinke is not the first member of the Trump Cabinet to come under scrutiny from the Justice Department. Earlier this year, according to two individuals with knowledge of the matter, the Environmental Protection Agency’s inspector general referred a case involving then-Administrator Scott Pruitt’s rental deal with lobbyist Steve Hart and his wife Vicki to federal prosecutors. The Justice Department inquiry, first reported by CNN on Tuesday, lost momentum after Pruitt resigned in July.

Zinke’s critics welcomed the idea of a potential Justice Department investigation and vowed to press for answers.

“If Democrats are given the opportunity to hold a congressional majority next year, Secretary Zinke will be called to testify in February 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,” said Rep. Raúl M. Grijalva (Ariz.), the top Democrat on the House Natural Resources Committee and one of the lawmakers who asked the inspector general to investigate the Montana land deal.

Jennifer Rokala, executive director of the advocacy group Center for Western Priorities, said in a statement that a Justice Department inquiry made sense given Zinke’s push for a new inspector general. “If Interior’s inspector general is unable to hold Secretary Zinke accountable without political interference, it’s time for career prosecutors at the Justice Department to take over,” she said.

Separately Tuesday, Democratic Sens. Richard Blumenthal (Conn.), Elizabeth Warren (Mass.) and Ron Wyden (Ore.) wrote Zinke urging him “to immediately cease any efforts to use the office of the Secretary of the Interior for personal gain and fully reimburse the public for your private use of public resources.” Citing the recent inspector general’s report, they questioned why Zinke invited former donors as official guests last year on a trip to California’s Channel Islands and asked his aides to look into designating his wife as a department volunteer.

Matt Zapotosky contributed to this report.