(CNN) -- Authorities arrested a Colorado man who allegedly sent a manila envelope stuffed with white powder to officials who had been handling his back-taxes case, federal prosecutors said Wednesday.
A haz-mat team tested the substance and discovered it was sodium bicarbonate, commonly known as baking soda, a statement from the U.S. Attorney's Office in Colorado said.
Matthew O'Neill, 51, was scheduled to appear in a federal court in Denver Wednesday afternoon. He faces a charge of false information and hoaxes related to a terrorism offense, federal prosecutors said.
Special agents from the FBI and the U.S. Postal Inspection Service arrested him Tuesday, six days after a manila envelope with his return address arrived in a mailroom at the Colorado Department of Revenue, the U.S. Attorney's Office said.
The investigation began after white powder spilled from the envelope onto the desk of a department employee. The worker had previously handled correspondence from O'Neill disputing claims that he owed more than $15,000 in back taxes, according to a criminal complaint against O'Neill.
The envelope also contained a letter "detailing his dispute to pay taxes" and 32 pages of "rhetoric associated with a sovereign citizen typically available on the internet," the complaint said.
If convicted, O'Neill faces up to five years in federal prison and up to a $250,000 fine.
According to the FBI, sovereign citizens are "anti-government extremists" who don't believe they need to adhere to state or federal laws.
"As a result, they believe they don't have to answer to any government authority, including courts, taxing entities, motor vehicle departments or law enforcement," the agency said last year.