(CNN) The man in charge of mine safety under the Trump administration is facing more scrutiny about his ties to an industry push to roll back safety regulations.

Democratic Sen. Sheldon Whitehouse of Rhode Island has sent a letter to the assistant secretary for mine safety and health, David Zatezalo, questioning his relationship with a coal mining company leader who has called on the White House to roll back several safety rules that Zatezalo told Congress he supports keeping.

Whitehouse's letter questions Zatezalo's testimony at his October confirmation hearing, where he said he supports the current level of enforcement of the mine dust safety rule meant to reduce black lung disease cases in coal miners -- and another rule that punishes companies with repeat safety violations.

That testimony, Whitehouse says, contradicts an "action plan" sent to the White House by Bob Murray, the CEO of Murray Energy Corp., whom Zatezalo has publicly called "one of the people whose opinion I value."

"Over the last several months, the public has learned that Mr. Murray has been working behind the scenes actively to promote policies that would benefit his company," Whitehouse's letter says, making public a letter from Murray Energy addressed to Vice President Mike Pence that lays out his wish list for regulation rollbacks.

A Department of Labor spokesperson told CNN that "no one gets special treatment" at the Mine Safety and Health Administration.

"Assistant Secretary Zatezalo restated his strong commitment to enforcement when he testified before Congress earlier this month," the spokesperson said.

A spokesperson for Murray Energy said the company CEO hasn't met or spoken with Zatezalo since his confirmation in November.

"This is a blatant attempt by Senator Whitehouse to make an issue out of nothing," the spokesperson said. "Indeed, Senator Whitehouse is one of the greatest enemies of reliable, low-cost electricity in the United States, and his actions against the United States coal industry are to the detriment of our citizens on fixed incomes, single mothers and manufacturers of products that must compete in the global marketplace."

CNN has previously reported on Zatezalo's history of opposing the safety rules he now is charged with enforcing.

When Zatezalo was the CEO of Rhino Resources -- a coal company where inspectors documented more than 1,100 safety violations since 2008 -- he initiated litigation on behalf of the Ohio Coal Association against the department he now runs, challenging a rule that goes after companies with a pattern of violations.

But in a congressional hearing last month, Zatezalo said his past work for mining companies does not present a conflict of interest and he has no plans to recuse himself from decision-making on the Ohio Coal Association case that challenges those safety rules.

Whitehouse's letter asks Zatezalo to clarify whether he's made any commitments to Murray Energy since he was confirmed or if he's changed his position that he "would not propose any reduction in the enforcement," as he said during his confirmation hearing.