ORLANDO — A boy from Kentucky riding Walt Disney World’s Rock ‘n’ Roller Coaster slumped over during the ride and later died though his Army Green Beret father attempted to resuscitate him.

There was no indication Thursday why Michael Russell, 12, of Fort Campbell, Ky., lost consciousness on the popular indoor roller coaster at Disney-MGM Studios, or why he died. The Orange County Sheriff’s Office is investigating the death, and the medical examiner’s office will conduct an autopsy todayfriday.

Michael’s parents, Byron and Charlotte Russell, and brother Houston, 7, were being assisted Thursday by a Disney official.

“Right now our deepest sympathies are with the family, and we are doing everything we can to assist them,” said Disney spokesman Jacob DiPietre.

Byron Russell is a member of the Army’s 5th Special Forces Group, stationed at Fort Campbell.

“The family is in mourning,” said Maj. Jim Gregory, a public affairs officer for the special force’s command.

Disney closed the ride and asked the Florida Bureau of Fair Rides Inspection to assist in an inquiry. There was no indication Thursday that any equipment on the ride malfunctioned.

The death is the seventh in 18 months involving people stricken at Walt Disney World attractions, compared with two such deaths in the previous 18 months, according to state records.

None of the other six deaths, dating to Christmas Eve 2004, involved accidents. In some of those cases, the autopsy found that person had pre-existing, dangerous medical conditions.

Disney’s high-speed rides, including Rock ‘n’ Roller Coaster, display signs warning away people with critical medical conditions.

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.

Rock ‘n’ Roller Coaster is the second-fastest ride at Disney, topping out at 57 miles per hour. It opened in July 1999 and features a high-acceleration launch that reaches maximum speed in 2.8 seconds, a rollover and a corkscrew turn. The trains are modeled after limousines and travel the 3,403-foot track in one minute and 22 seconds.

The ride is themed after the rock band Aerosmith, and the band’s music provides a soundtrack.

At its peak it produces a pressure on the body that is 5 times that of gravity, a rate considered high but not unusual for roller coasters. Ride safety consultant Ken Martin of Virginia said 5 Gs is sufficient to cause some people to lose consciousness.

“Five Gs, depending on the length [duration] of those Gs, is pretty tough,” said Martin, a Virginia ride safety inspector.

Disney restricts the ride to people at least 48 inches tall.

Disney said 36.6 million people have ridden Rock ‘n’ Roller Coaster since its 1999 debut.

There have been no previously reported deaths, nor any major injuries or illnesses since at least 2002, according to available state records. A Web-based organization that monitors theme park news internationally, themeparkinsider.com, cites a 2000 incident in which a 43-year-old man suffered a brain bleed on the ride.

Employees at Disney-MGM Thursday told visitors the closed ride had technical difficulties, though they said they did not know what was wrong with it. Many visitors were unaware of the boy’s death. When informed, they were saddened, though many said they still wanted to ride Rock ‘n’ Roller Coaster.

“This breaks my heart. It really worries me now about roller coasters,” said Rene Boylan, 41, of Easton, Pa., whose 7-year old son had ridden the roller coaster and wanted to ride again. “I might start rethinking some of these rides for him. The roller coasters seem to be going to extremes now. I might be more cautious as to what I let him go on.”

But like many people Thursday, she said she is confident Disney runs safe rides.

“I can’t say they’re responsible,” she said. “Accidents do happen. I don’t fault the ride.”