The U.S. Department of the Interior confirmed it launched a preliminary investigation into reports that Interior Secretary Ryan Zinke attempted to pressure Republican Sens. Lisa Murkowski and Dan Sullivan of Alaska following their votes on health care last month.

The New York Times’ Eric Lipton tweeted a copy of a letter sent by Deputy Inspector General Mary L. Kendall to Rep. Frank Pallone Jr. (D-N.J.), who’s on the Committee on Energy and Commerce, and Rep. Raul Grijalva (D-Ariz.), who’s on the Committee on Natural Resources. She wrote that her office was undertaking a preliminary investigation into Zinke’s phone calls to the Alaska senators.

Last month, Sullivan told the Alaska Dispatch News that Zinke made it clear that his call was in response to Murkowski’s opposing vote on health care. He added the call carried “a troubling message,” and he worried some of Alaska’s state-specific projects, particularly those pertaining to energy, were in jeopardy.

Interior IG confirms it's investigating Sec Ryan Zinke's apparent (failed) effort to pressure Sen Murkowski to vote yes on Obamacare repeal pic.twitter.com/isLnpUyDMX — Eric Lipton (@EricLiptonNYT) August 23, 2017

Murkowski also serves as the chair of the Senate Committee on Energy and Natural Resources.

A spokesperson at the committee for Murkowski said the Alaska senator did not take Zinke’s call as a threat. She added, “While that was a private call, and the discussion will stay between them, they remain on good terms. ... To be honest, we are not clear what the IG’s office is ‘preliminarily investigating.’”

Murkowski was one of two only Republican senators, along with Susan Collins of Maine, to repeatedly break party lines as the GOP tried to make good on its promise to repeal and replace the Affordable Care Act.

Trump publicly derided Murkowski on Twitter, saying she had let her state and nation down by voting no. The Alaska senator seemed unfazed by the attack, telling the press she was in Washington to govern, not to constantly campaign for re-election.

Sen. John McCain (R-Ariz.) cast the deciding vote opposing the Health Care Freedom Act, joining Murkowski and Collins and ultimately derailing the GOP’s hopes to repeal parts of Obamacare.

This post has been updated with a statement from a spokesperson on behalf of Murkowski.