Washington (CNN) President Donald Trump announced Saturday morning that Interior Secretary Ryan Zinke will depart from his administration at the end of the year.

"Secretary of the Interior @RyanZinke will be leaving the Administration at the end of the year after having served for a period of almost two years. Ryan has accomplished much during his tenure and I want to thank him for his service to our Nation," Trump wrote in a tweet.

The President continued in a second tweet: " ... The Trump Administration will be announcing the new Secretary of the Interior next week."

The departure comes amid multiple ethics investigations into Zinke as well as a staff shake-up among Trump administration officials , including the President's chief of staff. Trump announced Friday that his budget director, Mick Mulvaney, will become his acting chief of staff, replacing John Kelly, at the end of the year.

The Washington Post reported Saturday that administration officials said the White House had been pushing Zinke to resign for weeks and that he was told he had until year's end to leave or he would be fired. Citing two people familiar with the discussion, The New York Times also reported that Kelly's team had told Zinke that he might risk a potentially humiliating firing if he didn't leave his post by the end of the year.

Some possible contenders for his position include Deputy Interior Secretary David Bernhardt and Nevada Republican Sen. Dean Heller, who lost his re-election race, sources with knowledge of the situation said.

Zinke said in a statement on Saturday that he was proud of his work as interior secretary and called the allegations against him "false."

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

I love working for the President and am incredibly proud of all the good work we've accomplished together. However, after 30 years of public service, I cannot justify spending thousands of dollars defending myself and my family against false allegations. Full statement attached. pic.twitter.com/gwo75SA6bM — Secretary Ryan Zinke (@SecretaryZinke) December 15, 2018

In a private resignation letter, Zinke blamed "vicious and politically motivated attacks" for his departure, The Washington Post reported.

The Justice Department is investigating Zinke for possibly using his office for personal gain following a referral from the Interior Department's inspector general, CNN has previously reported.

The Interior inspector general has multiple inquiries into the secretary, including the department's handling of a Connecticut casino project, whether the boundaries for Grand Staircase-Escalante National Monument were redrawn to benefit a state lawmaker, and conversations between Zinke and Halliburton Chairman David Lesar about a land development project in Zinke's hometown of Whitefish, Montana.

"I follow all rules, procedures, regulations and most importantly the law," he told CNN. "This is another politically driven investigation that has no merit."

Since assuming office in March 2017 , Zinke has worked closely with Trump on a number of policy fronts, including moves toward shrinking several expansive tracts of land designated national monuments by the Obama administration, a designation that restricts the types of recreation and energy development allowed on the land.

Zinke launched an ambitious energy development agenda, including a reassessment of the entire nation's offshore energy potential.

He also deployed Interior Department resources to federal lands along the US-Mexico border, which he said in a November statement had resulted in a "4,000% increase in arrests of illegal aliens" by law enforcement within his department.

Zinke often mentioned the President's experience as a builder when championing an effort to secure funding to rebuild aging infrastructure at national parks and other sites.

Environmental groups were quick to applaud Zinke's pending departure.

"Zinke's days of plundering our lands and enriching himself and his friends are over," Nicole Ghio, senior fossil fuels program manager for Friends of the Earth, said in a statement. "With an average of nearly one federal investigation opened into his conduct in office per month, Zinke's highly questionable ethics have finally caught up with him. Now, he is just another name on Trump's list of disgraced cabinet officials, which the Republican-led Congress has failed to hold accountable."

Senate Minority Leader Chuck Schumer also released a statement blasting Zinke.

"Ryan Zinke was one of the most toxic members of the cabinet in the way he treated our environment, our precious public lands, and the way he treated the government like it was his personal honey pot," Schumer, D-New York, said. "The swamp cabinet will be a little less foul without him."

Tennessee Republican Sen. Lamar Alexander, who sits on the Senate Energy and Natural Resources Committee, praised Zinke's work on legislation aimed at addressing a maintenance backlog at the nation's parks through the use of energy lease revenues, writing on Twitter that the bill "could do more to restore national parks than anything that has happened in the last half century."

"The bill will cut in half the nearly $12 billion deferred maintenance backlog in our parks, and Secretary Zinke has visited the Great Smoky Mountains National Park twice to discuss the importance of the bill," Alexander said. "I'm grateful for his service and appreciative of the work he has done to help rebuild and restore our parks for generations to come."

Montana Republican Sen. Steve Daines, who is also a member of the panel, thanked Zinke for his work as well.

"Thank you @SecretaryZinke for restoring commonsense management of our public lands, fighting to end the war on coal and for making the U.S. energy dominant. Montana is proud of you!" he wrote on Twitter.

The departure from Interior may not be Zinke's final turn in the spotlight. A former congressman, state legislator and Navy seal, Zinke had been mentioned as a potential candidate for Montana governor.

A source familiar with the situation, however, told CNN that they do not believe Zinke is interested in running for governor, but that it was time for Zinke to transition.

One of the potential factors was the time that Democrats in the House might spend probing the Interior secretary, calling him before the House Natural Resources Committee, and the distraction that could cause for the department, the source said. The committee's incoming chairman, Rep. Raul Grijalva, called for Zinke's resignation in a USA Today op-ed last month, prompting Zinke to attack the Arizona Democrat's character in a post on Twitter.

The source emphasized that it was not that Zinke was afraid of defending himself, but that the staff resources, time and preparation it would take to prepare for such showdowns could distract from the agency's work.

Keeping open the possibility of investigations into Zinke by the committee, Adam Sarvana, Grijalva's spokesman, told CNN on Saturday that "it is safe to say that our oversight of former Secretary Zinke has not even begun yet."

"We will extend the invitation to testify and take it from there," Sarvana said, adding that questions for Zinke would include decisions he made while at the department, including over national monuments and oil and gas leasing.

Earlier Saturday, Grijalva said in a statement that he hopes for a "genuine turning of the page" following Zinke's departure.

"This is no kind of victory, but I'm hopeful that it is a genuine turning of the page," he said. "Secretary Zinke's successor has a chance to move on from an unfortunate Trump Administration record of environmental mismanagement and decline. A well-managed Interior Department — one that puts the public good ahead of fossil fuel and mining industry demands — can be a boon to the entire country."

"The next Interior Secretary should respect the American people's desire for strong environmental standards and an end to corporate favoritism," Grijalva said. "The Democratic majority on the Natural Resources Committee will be ready to assist in that effort starting in January."