Former Trump official Ryan Zinke files paperwork for congressional seat in Montana

Former Interior Secretary Ryan Zinke, who served under Donald Trump and resigned in 2018 amid multiple ethics investigations, filed paperwork on Thursday afternoon to run for Montana's soon-to-be-established 2nd Congressional District.

Montana was allotted an additional district as a result of population growth tracked in the 2020 Census. Zinke, a Republican, previously represented Montana's at-large district from 2015 to 2017.

Zinke resigned amid investigations into his use of chartered planes, though the Interior Department's watchdog later said those flights did not break any rules, but some of the costs could have been avoided.

Zinke's use of charter or military planes was first confirmed amid questions about spending on travel for multiple cabinet officials in 2017 when Trump fired former Health and Human Services Secretary Tom Price after it was reported that he spent more than $1 million on travel, including private flights.

The Interior Department's inspector general found that a number of flights on private or military planes taken by Zinke generally followed the relevant rules and were all approved by ethics officials in advance. But the report found that the cost of at least one of the trips, a more than $12,000 flight from Las Vegas to Montana, could have been avoided.

Investigations into Zinke also included a probe into his involvement in a real estate deal with an executive in his hometown and his decision to reject a casino in Connecticut that was sought by two tribes. House Democrats called the actions "giveaways" to the oil and gas industry under Zinke's Interior Department.

In response to a tweet from Trump announcing his resignation in December 2018, Zinke said on Twitter he couldn't justify the cost of defending himself in a legal battle.

"I cannot justify spending thousands of dollars defending myself and my family against false allegations," he said at the time.

After his resignation, Zinke joined a Washington lobbying firm, Turnberry Solutions, to lobby on Capitol Hill.

ABC News' Stephanie Ebbs contributed to this report.