By Keith Coffman
DENVER (Reuters) - The leader of anti-government extremists known as the Montana Freemen, who clashed with federal authorities in the 1990s, died on Tuesday of apparent natural causes at the U.S. government's highest-security prison.
Leroy Schweitzer, 73, was found unresponsive in his cell early on Tuesday, according to a release from the Federal Correctional Complex in Florence, Colorado, where he was serving a 22-year sentence for various tax, fraud and weapons offenses.
"Medical assistance was immediately summoned, and efforts to resuscitate him were unsuccessful," the statement said.
An autopsy has been ordered, but prison officials said they did not believe his death was suspicious.
Schweitzer and his followers proclaimed the U.S. government to be illegitimate, declared themselves a sovereign state and openly defied federal laws.
Following his arrest in 1996, other Freemen engaged in an armed standoff with federal law enforcement at a fortified ranch outside rural Jordan, Montana. The confrontation ended peacefully after 81 days.
Schweitzer was convicted in 1998 by a federal jury in Montana of conspiring to commit bank fraud, threatening a government official, submitting false insurance claims, failing to file federal tax returns and firearms violations.
He was sentenced to more than 22 years in prison.
In 2003, two men posing as "Montana Marshals" were arrested after they attempted to free Schweitzer from a federal prison in South Carolina.
Schweitzer had been incarcerated at the Florence Complex, known as "Supermax," since 2006.
Supermax is home to some of the federal government's most infamous convicts, including Unabomber Ted Kaczynski, Oklahoma City bombing conspirator Terry Nichols, 1993 World Trade Center bomber Ramzi Yousef and shoe bomber Richard Reid.
(Editing by Steve Gorman and Cynthia Johnston)