(AP) – A former Republic Airlines pilot has pleaded guilty to threatening to torture and kill the company’s CEO and his family.

The U.S. attorney’s office says 37-year-old Matthew Richard Walker, of Faribault, Minn., entered the plea Monday in federal court in Kansas City.

Prosecutors said one threatening letter was discovered in August 2012 in the cockpit of an aircraft that was parked at Reagan National Airport in Washington. Four identical letters were later discovered on aircraft parked at Reagan International Airport, in Charlotte, N.C., and at Kansas City International Airport, as well as aboard an aircraft en route from Reagan National Airport to Indianapolis International Airport. A fifth letter was discovered in the KCI crew room.

Walker faces up to five years in federal prison without parole and a $25