WASHINGTON – Ryan Zinke, the embattled secretary of the Interior, is leaving the administration at the end of the year, President Donald Trump announced via tweet Saturday.

"Ryan has accomplished much during his tenure and I want to thank him for his service to our Nation," Trump said on Twitter. Zinke's replacement will be announced next week, he said.

In a statement on Twitter, Zinke said the mounting pressure had taken its toll, personally and financially.

"I love working for the President and am incredibly proud of all the good work we’ve accomplished together," he tweeted. "However, after 30 years of public service, I cannot justify spending thousands of dollars defending myself and my family against false allegations."

Zinke is the latest top-level Trump adviser who will be leaving the administration within the coming weeks, joining Chief of Staff John Kelly and United Nations Ambassador Nikki Haley.

More:The controversies that defined Ryan Zinke's tenure as Trump's Interior Secretary

More:Who's been fired and who resigned from the White House

Cabinet officials who have already left include Jeff Sessions, attorney general; Rex Tillerson, secretary of state; Dave Shulkin, secretary of veterans affairs; Tom Price, secretary of health and human services; and Scott Pruitt, administrator of the Environmental Protection Agency.

A former Montana congressman, Zinke, 57, has been embroiled in several investigations. In one case, the Interior Department's own inspector general reportedly referred Zinke to the Department of Justice for potential prosecution.

That case involves Zinke’s role in a Whitefish, Montana, land development deal, according to The Washington Post, which also reported Trump was concerned about the situation.

Earlier this fall, Trump told reporters he was evaluating Zinke’s future in the administration in light of the allegations.

Asked by reporters last month whether he might fire Zinke, Trump said, “No, I’m going to look into any complaints.”

A request for comment from a Zinke spokesperson Saturday was not immediately returned. His likely replacement, at least on an acting basis, would be Deputy Secretary David Bernhardt, a former oil lobbyist who's already drawn fire from environmental advocates.

Zinke's departure comes only weeks before Democrats take over control of the House after flipping 40 GOP seats in the midterm elections. Armed with subpoena power, Democratic leaders had said they planned to hold Zinke accountable for administration moves to reduce the size of public monuments, scale back protections on endangered species and expand off-shore drilling.

‘Shameless handmaiden’

Nancy Pelosi, likely incoming House speaker, called Zinke a “shameless handmaiden for the special interests.”

"His staggering ethical abuses have delivered a serious and lasting blow to America’s public lands, environment, clean air and clean water," she said.

Two weeks ago, after Rep. Raul Grijalva, D-Ariz., the top Democrat on the House Natural Resources Committee, called on Zinke to resign for ethical missteps, the secretary accused the congressman of being a drunk.

“It’s hard for him to think straight from the bottom of the bottle,” he wrote from his official Twitter account on Nov. 30.

Kieran Suckling, executive director of the Center for Biological Diversity, said Zinke "will go down as the worst Interior secretary in history.”

“His slash-and-burn approach was absolutely destructive for public lands and wildlife," Suckling said in a statement on Saturday.

The Arizona-based nonprofit center that supports the protection of endangered species has accused Zinke, among other things, of reducing protection for public lands and wildlife without public input.

But Zinke had his defenders.

Utah GOP Rep. Rob Bishop, who chairs the Natural Resources Committee, said the interior secretary had provided badly needed change from the Obama administration.

"Under his tenure, Secretary Zinke has moved the public lands conversation to how we manage our resources for the greatest benefit of the American people," Bishop wrote in a recent column for USA TODAY. "He has promoted an aggressive, forward-looking path to reform. This agenda focuses on enhancing public access to federal lands, modernizing land resource management, and streamlining decision-making."

Zinke’s record

Zinke, a former Navy SEAL, wielded an unflinching demeanor as the head of a sprawling agency. The Interior Department has roughly 70,000 employees, manages the country’s natural resources on land and offshore, and oversees federal lands that collectively make up a fifth of the country.

He also made addressing the nearly $12 billion national park maintenance backlog a priority. Still, an agency proposal last year to double many fees at 17 of the nation's most-visited parks during peak season died after fierce public opposition.

He often clashed with lawmakers.

Usually it was with Democrats over ethical issues or Trump's energy and environmental policies, such as the shrinking of national monuments. Grijalva, the Arizona congressman, said Zinke's agency routinely "ignored" his requests for information and documents regarding the department's decisions.

But he also caught heat in late 2017 for pressuring both of Alaska's senators – Republicans Lisa Murkowski and Dan Sullivan – to support an upcoming vote on an Obamacare repeal bill. His lobbying effort was viewed as a threat that the state – more than half of which is federal land – would somehow suffer if they opposed it.

Murkowski ended up opposing the bill, which was narrowly defeated, but she and Zinke shared beers to show there were no hard feelings.

Zinke ran the agency with what some viewed as a heavy hand.

A survey released in August of federal scientists and researchers across several agencies, including Interior, said that under the Trump administration, political concerns outweigh scientific rigor and budget cuts hamper their mission.

One survey respondent from the U.S. Geological Survey mentioned an Interior Department directive requiring a political appointee to review research grants of $50,000 or more to make sure they align with Secretary Ryan Zinke's priorities. That "impedes new and ongoing research," the respondent said.

A spokeswoman for Zinke at the time said: "Asking to ensure that discretionary grants aren’t used for frivolous purposes is sound management, not politics."

Zinke was also at the center of other controversies, including:

Travel: The IG admonished Zinke for his use of military charters, including one in June 2017 that cost $12,375 for a trip to speak at the developmental camp for the Golden Knights, a professional hockey team based in Las Vegas. He also was criticized by lawmakers for spending more than $53,000 on three helicopter trips in 2017, including one that returned him to Washington in time to take a horseback ride with Vice President Mike Pence, according to The Associated Press.

Office remodeling: Zinke drew criticism earlier this year for moving forward with the replacement of three sets of doors at Interior's historic headquarters, costing nearly $139,000. A spokeswoman for Zinke said at the time he was unaware of the contract before reporters started asking about it. The work is part of a decade-long modernization of the 1936 building that began before Zinke took office in March 2017.

Offshore drilling exemption for Florida: Zinke made a big deal in January when, after the president unveiled his plan to open up 90 percent of the Outer Continental Shelf off the U.S. coast to oil and gas exploration, he flew to Florida and revealed the Sunshine State would be exempt from offshore drilling. It was seen as a political gambit and a huge gift to Florida Gov. Rick Scott, who weeks later decided to challenge Sen. Bill Nelson, D-Fla. But The Washington Post later reported the move caused some friction within the White House because it was not coordinated with the West Wing's political shop.

Contributing: Associated Press