Representative Matt Shea, a Republican representing Washington's 4th legislative district, was a leader in the Patriot Movement defying federal authority in Nevada, Idaho and Oregon in 2014, 2015 and 2016, according to a report by the private Rampart Group commissioned by the Washington state House of Representatives and released on Thursday.
One of those actions involved a 41-day standoff at an Oregon wildlife refuge that resulted in one death, the report said.
"Representative Shea participated in an act of domestic terrorism against the United States by his actions before and during the armed takeover and standoff at the Malheur National Wildlife Refuge," the report said, using a definition under U.S. law.
The Republican House leadership immediately called for Shea's resignation, suspended him from the party caucus and removed him from his ranking position on the House Environment and Energy Committee.
The chief clerk of the House referred the matter to the FBI and U.S. attorney's office, J.T. Wilcox, leader of the minority Republicans in the statehouse, said in a statement.
"He should resign," Wilcox said separately on Twitter.
But Shea defiantly vowed to remain in his post, comparing the effort to oust him to the impeachment of U.S. President Donald Trump by Democrats in the U.S. House of Representatives.
"I will not back down, I will not give in, I will not resign," Shea, who declined to cooperate with the investigation, said on Facebook.
The right-wing militia movement expresses deep mistrust of the federal authority and disdains the regulation of firearms.
In 2014, Shea participated in an armed standoff in Bunkerville, Nevada, led by rancher Cliven Bundy, who insisted on the right to let his cattle graze on federal lands without paying fees, the report said.
In 2015, the report said, Shea engaged in another conflict in Priest River, Idaho, in support of a military veteran who defied a lawful order to surrender his weapons.
In 2016, Shea aided in the planning of another uprising by Ammon Bundy, the son of Cliven Bundy, who took over a national wildlife refuge in Harney County, Oregon, for 41 days.The report cited other instances of Shea condoning violence, intimidating political opponents and promoting militia training for armed conflict against law-enforcement agencies.