A head scientist at the Interior Department's U.S. Geological Survey (USGS) resigned in December following concerns about the department's interference in the scientific process.

In a letter obtained and released by Mother Jones on Wednesday, head of USGS's energy and minerals program Murray Hitzman resigned from his role at the end of last year in "protest" after the department provided final results of a study he had been working on to Interior Secretary Ryan Zinke Ryan Keith ZinkeTrump extends Florida offshore drilling pause, expands it to Georgia, South Carolina Conspicuous by their absence from the Republican Convention Trump flails as audience dwindles and ratings plummet MORE days before it was to be made publicly available, an action he called in the letter a "contradiction of my interpretation of USGS fundamental scientific policy."

The study in question was a resource assessment of the nation's petroleum reserve in Alaska. USGS led the onshore efforts of the joint interagency assessment.

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According to an Interior press release, USGS estimated a mean of 8.7 billion barrels of oil and 25 trillion cubic feet of gas in Alaska soil. The release called it "a significant increase​ from the 2010 resource assessment, which estimated a mean of 1.5 billion barrels of oil."

The report was released on Dec. 22. Hitzman gave his resignation six days prior on Dec 17.