10:00 AM PT — TMZ has learned that the passenger has unfortunately died … their body has been turned over to the King County Medical Examiner.

A United Airlines passenger suffered a medical emergency Tuesday after he turned “deep purple” during a flight from Tokyo to Denver … and the plane had to be diverted to Seattle for an emergency landing, according to witnesses and officials.

The horrific incident began about 6 and a half hours into the 10 and a half hour flight from Narita International Airport in Japan to Denver International Airport in Colorado, according to an eyewitness and the Federal Aviation Administration. The FAA said the crew reported that a passenger was having a medical emergency.

The eyewitness tells TMZ … she was woken up by a commotion near her seat and was shocked to see an unconscious man lying on the floor with 3 firemen and a stewardess surrounding him.

According to the witness … the firemen, who were also passengers on the flight, were pumping fluids into the man’s body with IV bags and using oxygen tanks to keep him breathing through a mask.

The witness said she went to talk to a flight attendant who told her they were going to make an emergency landing at the closest airport, which was Seattle-Tacoma International Airport.

At one point, the witness said, the pilot came out of the cockpit to assess the situation … and one flight attendant was overheard telling another that they had 45 minutes until they were scheduled to land in Seattle.

All the while, the firemen were working to save the unconscious man as his family looked on only to see his skin turn “deep purple,” according to the witness. After they finally landed, one of the firemen hooked the man’s arm with his own arm and dragged him down the aisle and out of the plane, the witness said.

A rep for United Airlines tells us …the aircraft, carrying 245 passengers and 12 crew members, was met by medical personnel upon arrival — and the flight eventually continued on to Denver.