Fumes from the gasoline caused an explosion. The fire caused more than $1 million in damage at MSU's Agriculture Hall. It also destroyed records related to the government's $2 million support for the research.
"There was no intention to harm any living thing," Mason's defense attorney, John Minock, said in an interview. "They didn't intend to cause an explosion. They just wanted to damage some paper."
Mason appeared in federal court in Grand Rapids to plead guilty to conspiracy and arson charges, including an incident that destroyed logging equipment in northern Michigan a day after the MSU fire.
U.S. Attorney Charles Gross has called it "domestic terrorism, plain and simple."
The government says Mason and Ambrose were affiliated with the Earth Liberation Front, a radical band of environmentalists. They were married at the time but divorced earlier this year.
Minock said prosecutors built their case against Mason with help from Ambrose, who tape-recorded their conversations in 2007.
Ambrose's attorney, Michael Brady, declined to comment. Assistant U.S. Attorney Hagen Frank could not be reached.
The explosion at MSU burned Mason's hair and prevented her from finishing the message, "No GMO," on a wall, a reference to genetically modified organisms. She and Ambrose fled the building and drove 190 miles north to Empire.
In her plea agreement, Mason also admits involvement in 12 other acts with property damage pegged at more than $2.5 million.
They include the destruction of four homes under construction in Washtenaw and Macomb counties and an attempted arson at an Ice Mountain bottled water pumping station in Mecosta County, all in 2003.
Mason will not be charged with those incidents, but they could affect her sentence. The U.S. attorney's office agrees not to seek more than 20 years in prison but could appeal if U.S. District Judge Paul Maloney goes below 15 years. No date was set.
A co-defendant, Stephanie Lynne Fultz, has agreed to plead guilty to failing to report the MSU arson to authorities, court documents say. The government says she cut Mason's scorched hair after the fire.
Aren Burthwick has agreed to plead guilty to a similar charge for helping Mason and Ambrose and not reporting the arson, documents say.
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