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WASHINGTON — Ryan Zinke, the secretary of the Interior Department and a key figure in President Trump’s push to roll back environmental regulations and ramp up oil drilling, is facing increased scrutiny amid federal investigations into allegations that he abused travel spending and maintained close ties with industries he oversees.

The criticism escalated sharply after reports this week that the Interior Department’s inspector general had referred one of the inquiries to the Justice Department, a potential prelude to a criminal investigation.

T​he​​ investigations have raised questions about Mr. Zinke’s oversight of the Interior Department, where he is essentially the largest land manager in the United States. He has authority over the nation’s 300 million acres of public lands as well as vast waters off the Atlantic and Pacific coasts and the Gulf of Mexico.

Mr. Zinke faces at least a half-dozen ethics investigations. At least nine other inquiries into alleged ethics violations have been closed, in some cases because Mr. Zinke was cleared, in others because of a lack of cooperation with investigators.