The sources said Zinke wanted to avoid grilling from Democrats, who are about to gain control of the House of Representatives, over ethics probes.
Zinke's alleged misconduct as Interior Secretary -- regarding incidents like threats to Senator Lisa Murkowski and improper use of chartered flights -- prompted 18 government investigations.
In November, Zinke feuded with Representative Raul Grijalva after the Congressman wrote an op-ed in USA Today calling for Zinke's resignation over a series of "ethical and managerial failings."
Zinke, a former Navy SEAL and Congressman from Montana, did not want to face the scrutiny or legal costs associated with facing the allegations, Bloomberg News reported.
Possible successors for Zinke include Deputy Secretary of the Interior David Bernhardt, former Wyoming Congresswoman Cynthia Lummis and Utah Attorney General Sean Reyes.
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