MIAMI -- Derrick Thomas' legs weren't red or swollen and his body temperature was normal.
There were no visible signs of a blood clot, doctors said Wednesday. But when Thomas was being moved from his hospital bed to a wheelchair on his way to therapy Tuesday morning, something triggered a massive blood clot in his pulmonary artery that provides blood and oxygen from the heart to the lungs.
Thomas, 33, told his mother he wasn't feeling well just before his eyes rolled back, said Dr. Frank Eismont, an orthopedic surgeon at Jackson Memorial Hospital.
The Chiefs star, who held the NFL record of seven sacks in a game, went into cardio-respiratory arrest. Thomas, paralyzed from the chest down in a car crash two weeks earlier, died minutes later.
The cause of death, confirmed Wednesday by an autopsy, is common for paralysis victims.
"A certain percentage of people with paralysis suffer from blood clots," said Dr. Barth Green, a neurosurgeon at Jackson Memorial Hospital. "Sometimes you can see them and sometimes they're hidden in the deeper veins of the body, and that's what happened with Derrick."
Between 3 percent and 15 percent of spinal cord injury patients suffer from blood clots, Green said.
Thomas was driving a car during a snowstorm on Jan. 23 as he and two friends headed to the Kansas City airport to fly to St. Louis for the NFC championship game. He lost control of the car, and it overturned several times.
Police said Thomas was speeding and weaving in traffic, but no charges were filed.
Thomas and passenger Michael Tellis, 49, were not wearing seat belts and were thrown from the car. Tellis was killed and Thomas' spine and neck were broken. The third person in the car was wearing a seat belt and received minor injuries.
Thomas was brought to the hospital in Miami, his hometown, where he had surgery to repair his spinal column.
The nine-time Pro Bowl linebacker had been on blood thinners since the accident, doctors said. Therapists also used mechanical devices on Thomas daily that intermittently contracted his legs to help circulate blood.
"We took all the aggressive precautions that are available to people today," Green said. "It just wasn't enough."
Added Eismont: "If I were to have it all to do over again, I wouldn't do anything different."
Thomas had been progressing quickly in rehabilitation. He was allowed out of the hospital last weekend -- although a nurse and a physician stayed with him -- and he had been talking about returning to Kansas City within the next few months, said Dr. Jon Browne, Chiefs team physician.
A news conference had been discussed that would have informed the public of Thomas' upcoming moves.
Thomas left behind thousands of grieving fans and nearly as many friends and relatives trying to cope with his sudden death. He will be remembered in both Kansas City, Mo., and Miami within the next 10 days.
Memorial services are set to begin in Kansas City and then move to Miami, Thomas' hometown and the place where he will be buried.
There will be a viewing Monday at Thomas' church in Kansas City, then a memorial service at Kemper Arena the following day, Chiefs president Carl Peterson said.
"I'm not sure it can hold all the people who want to attend," Peterson said. "It will be a public opportunity for Derrick's fans and friends to honor him."
Thomas will be transferred back to Miami, where there will be a viewing Feb. 18. He will be buried Feb. 19.
The reason for the delay is because Thomas' aunt, Edith Morgan's sister, died last weekend, Peterson said. Thomas' family will be at her funeral Saturday in Tallahassee.
A week before he died, Thomas dictated a thank you letter for the outpouring of support he and his family had received since the accident. It will be released soon, Peterson said.
An All-American at Alabama, the 6-foot-3, 255-pound Thomas became an immediate star as a pass-rushing specialist after being taken in the first round of the 1989 draft. He was an All-Pro in his first nine seasons and ranked ninth on the career list with 126´ sacks.
He set the sack record of seven against Seattle in 1990. That game was on Veterans Day, and Thomas dedicated his performance to his father, an Air Force pilot killed in Vietnam.
Thomas is survived by his mother; his son, Derrick Thomas Jr., 8; and a half-brother, Gregory Morgan, 19.
"He's really one of everyone's own," Peterson said. "He's a special, special guy that came our way. And we were very, very fortunate to have him 11 of his 33 years. You don't replace someone like this. We won't forget him."