What was meant to be a mission to save the life of a man in desperate need of a lung transplant ended instead on Monday in the deaths of four members of a medical team and two pilots of a plane that crashed while racing to fly the organs from Milwaukee to Michigan.

Investigators from the National Transportation Safety Board said Tuesday that they thought mechanical difficulties, not a light rain or wind, caused the plane, an eight-seat Cessna jet, to slam into Lake Michigan at about 190 miles an hour on Monday afternoon, just minutes after taking off from General Mitchell International Airport in Milwaukee.

Besides the two pilots, the accident killed two transplant surgeons and two perfusionists — specialists in transplanting organs — from the University of Michigan Medical Center.

Hospital officials vowed that their transplant program would not be interrupted, despite the loss to their transplant team. The surgeons were among 15 transplant surgeons at the University of Michigan. The perfusionists were among three full-time perfusionists.

The crash highlighted the inherent dangers for air ambulance crews, under pressure from transplant programs to move quickly when an organ becomes available. Both the safety board and the Federal Aviation Administration began scrutinizing medical flights several years ago after an increase in accidents.

“For the flight crews who are doing this routinely, they have a very high death rate,” Dr. Baker said. “It’s a very high-risk occupation.”

I did not realize this was this was a common occurrence. This is really sad.