A woman flying from Chicago to San Francisco died while in transit this weekend, after her "medical emergency" forced the plane to make an unplanned stop in Montana.

According to the Billings Gazette, the United Airlines 737 left Chicago at 7 p.m. Saturday night, and was headed to San Francisco International Airport.

At 9:30 p.m., as the plane passed over South Dakota, air traffic controllers at Montana's Billings Logan International Airport were notified that the plane would be landing there due to a passenger's medical emergency.

The flight landed at 10 p.m., where it was met by emergency crews and paramedics.

When contacted by SFist, a spokesperson with the Yellowstone County Medical Examiner & Coroner says that 42-year-old passenger Tina Matthews of Antioch was pronounced dead at the scene.

Passengers on the flight deplaned while the aircraft was inspected and refueled. They then reboarded, and by midnight were back in the air. They landed at SFO at around 1:20 a.m. Sunday.

As of Monday morning, the cause of Matthews' death had not been released. According to the Coroner's spokesperson, an autopsy on her remains has been scheduled.