A doctor who was running as a Libertarian candidate for U.S. Senate has died in a plane crash.

The Dubuque County Sheriff's Office says Dr. Douglas Butzier was the pilot of the plane that crashed around 11 p.m. Monday near Dubuque Regional Airport. He was the sole occupant of the aircraft.

Butzier worked at Mercy Medical Center-Dubuque. Jennifer Faley, a hospital spokeswoman, confirms Butzier was running for U.S. Senate against Democratic U.S. Rep. Bruce Braley and Republican state Sen. Joni Ernst.

Federal Aviation Administration spokesman Lynn Lunsford says the plane took off from Ankeny Regional Airport about an hour before thecrash. The National Transportation Safety Board will investigate the crash.

The party's candidate for governor, Dr. Lee Hieb, says Butzier just last weekend had flown her and several Libertarian Party candidates around the state to campaign.

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Hieb, an orthopedic surgeon, says she talked medicine with Butzier, a Dubuque emergency room surgeon, during some of their campaign events. He told her he became a Libertarian after studying economics for a Master of Business Administration about seven years ago.

She says "Iowa and the cause of individual liberty are significantly diminished by his passing."