The Russell family told Orange County sheriff's deputies that Michael was in good health.
"There are no previous medical problems they know of," said Orange County sheriff's Detective Mark Hussey, who was assigned to investigate the death. "He said his son had a yearly physical, the most recent just a few months ago."
The Russells boarded Rock 'n' Roller Coaster after 11 a.m., with Michael sitting next to his mother and brother and his father behind them, said Deputy Barbara Miller, a sheriff's spokeswoman.
"The dad noticed the little boy went limp as the ride finished," Miller said.
Deputies say Byron Russell immediately began CPR on his son, and a ride employee called 911 at 11:20:36 a.m., summoning paramedics from the Reedy Creek Fire Rescue.
Paramedics stationed at the park arrived about three minutes after the 911 call first came in and took over, according to Bo Jones, Deputy Chief of Reedy Creek Fire Rescue. Though Disney has portable heart defibrillator units stationed throughout its parks, there was not one available to use on Michael until the paramedics arrived, according to an audiotape of the 911 call. (Paramedics earlier indicated it took about six minutes for them to arrive.)
The child's heart was first shocked 7 minutes and 3 seconds after the 911 call, according to Jones.
The boy was transported to Celebration Hospital where he was pronounced dead.
Portable defibrillators, called automatic external defibrillators, determine if a heart is quivering without pumping blood and will automatically shock it to try to restore a normal beating pattern. The procedure is not always successful.
The availability of portable defibrillators at Walt Disney World became an issue earlier this month when a wrongful death lawsuit was filed by the parents of a child who died last year after riding Mission: Space at Epcot.