Interior Secretary Ryan Zinke is under investigation by his own agency's inspector general over reports that he threatened Sen. Lisa Murkowski, R-Alaska, with repercussion for her healthcare vote last week.

The Interior Department's Office of the Inspector General said Thursday that it has started a "preliminary investigation" into the reports after two senior House Democrats filed a formal petition with the IG and the Government Accountability Office a week ago to determine if Zinke violated anti-lobbying laws.

"The OIG is undertaking a preliminary investigation into this matter," Deputy Inspector General Mary Kendall stated in a letter to Energy and Commerce Committee ranking member Rep. Frank Pallone, D-N.J., and Natural Resources Committee ranking member Rep. Raul Grijalva, D-Ariz. "We will advise you about what further action the results of this inquiry lead the OIG to take."

The investigation was announced just hours after Zinke tweeted a photo of him and Murkowski having beers together, calling her a friend.

Sen. Dan Sullivan, R-Alaska, told news outlets last week that he and Murkowski were called by Zinke and told that there would be repercussions for their state because of her vote against a procedural measure to move ahead with an Obamacare reform bill.

Sullivan said Zinke's message was "troubling," according to the Alaska Dispatch News. "I'm not going to go into the details, but I fear that the strong economic growth, pro-energy, pro-mining, pro-jobs and personnel from Alaska who are part of those policies are going to stop," he said.

Sullivan told the newspaper he tried to "push back" on behalf of the Alaskan people, saying the state is "facing some difficult times and there's a lot of enthusiasm for the policies that Secretary Zinke and the president have been talking about with regard to our economy. But the message was pretty clear."

President Trump called Murkowski out on Twitter for standing with the Democrats in opposing the procedural measure. The vote was 51-50, with Vice President Mike Pence casting the tie-breaking vote.

"Senator @lisamurkowski of the Great State of Alaska really let the Republicans, and our country, down yesterday. Too bad!" the president tweeted.