David Bernhardt, the Interior Department’s No. 2 official and a former fossil fuel lobbyist, took over Wednesday as acting secretary after the resignation of scandal-plagued agency chief Ryan Zinke.

With Democrats on Thursday taking back control of the House of Representatives — and the powerful subpoena authority that comes with it — they have vowed to investigate Zinke’s conduct and policy decisions.

Zinke, a former Montana congressman and Navy SEAL, faced nearly 20 federal investigations. Several are ongoing, and one has been referred by Interior’s internal watchdog to the Justice Department for possible criminal violations. He has maintained he did nothing wrong and blamed his departure on “vicious and politically motivated attacks.”

By midday Wednesday, his Twitter profile read “former secretary.” In a farewell message posted to the social media site, he said it was a “high honor to serve @POTUS & the American People.”

“We’ve restored public lands ‘for the benefit & enjoyment of the people,’ improved public access & shall never be held hostage again for our energy needs,” Zinke wrote.

It's been a high honor to serve @POTUS & the American People as @Interior Secretary. We've restored public lands “for the benefit & enjoyment of the people,” improved public access & shall never be held hostage again for our energy needs. God bless America & those who defend her. pic.twitter.com/JXzVmrpDTg — Secretary Ryan Zinke (@SecretaryZinke) January 2, 2019

Trump has not nominated a permanent replacement, as he said he would do in a post to Twitter last month. But several names have surfaced as contenders for the role, including Rep. Rob Bishop (R-Utah) and outgoing Sen. Dean Heller (R-Nev.). Whoever Trump taps must be confirmed by the Senate.

Before being sworn in at the Interior Department on Aug. 1, 2017, Bernhardt, a Colorado native, worked for eight years at the law firm Brownstein Hyatt Farber Schreck, where he lobbied on behalf of oil, gas, mining and agricultural interests. He also previously served as a top Interior official under former President George W. Bush, leading an attempt to open Alaska’s pristine Arctic National Wildlife Refuge to oil and gas development. (Last month the Trump administration took a significant step toward its goal of allowing drilling in the refuge, which Zinke said “stands out among the most impactful” Interior accomplishments in terms of “bolstering America’s economic strength and security.”)

The Senate confirmed Bernhardt as Interior’s deputy secretary in July 2017. Before his confirmation hearing, 150 conservation groups signed a letter opposing his nomination, citing his ties to the industries he is now charged with regulating and labeling him “a walking conflict of interest.”

He has so many potential conflicts that he carries around a list of former clients and is barred from participating in decisions affecting them, The Washington Post reported in November.

Bloomberg via Getty Images President Donald Trump and acting Interior Secretary David Bernhardt (left) at a meeting in the Cabinet Room of the White House on Jan. 2, the first day Bernhardt was in charge of the department.

As Interior’s deputy secretary, Bernhardt met on several occasions with lobbyists for MGM Resorts International, the casino-resort giant that his longtime former employer represents, as HuffPost previously reported. The ethics agreement he signed last year bars him from participating in matters involving Brownstein Hyatt Farber Schreck.

Last year the Interior Department blocked a proposed casino partnership between two Connecticut tribes. The project would have been competition for an MGM casino in nearby Springfield, Massachusetts, and the company lobbied against federal approval. Interior’s decision is under investigation by the agency’s inspector general.

Bernhardt played a key behind-the-scenes role in many regulatory rollbacks during Zinke’s tenure, as Mother Jones highlighted in an October profile. Those efforts include a plan to dismantle Obama-era protections for the greater sage grouse, a move that would open up millions of acres of the declining game bird’s habitat to energy and mineral development. Bernhardt is also leading a charge to revise the Endangered Species Act, one of America’s bedrock conservation laws. The proposed changes would make it easier to remove recovered species from the protected list and would change how federal agencies go about designating habitat as critical to the long-term survival of species.

After news of Zinke’s resignation, the liberal environmental group Western Values Project launched a website detailing Bernhardt’s work with special interests. In an accompanying statement, the nonprofit’s executive director, Chris Saeger, called Bernhardt the “ultimate DC swamp creature.”

“The bottom line is that Bernhardt is too conflicted to even be Acting Secretary,” Saeger said.

Bernhardt represented Interior at Wednesday’s Cabinet meeting at the White House, sitting directly next to Trump.