A paraglider who was killed in an accident on Mount St. Helena Sunday died of multiple blunt impact injuries and was likely airborne for only a few minutes, a Napa County Sheriff's Office spokeswoman said Tuesday.

Walden Grindle, 35, of St. Helena died after crashing his speed glider, a smaller version of a paraglider-type parachute, on the south slope of the mountain around midday. He was able to activate an emergency beacon and call his wife, the Napa County Sheriff's Office said, but rescuers found him dead when they arrived a short time later.

Grindle's speed glider was still attached to his body when rescuers reached him, said Sheriff's Office Capt. Tracey Stuart, who released results of an autopsy Tuesday. She said officials are speculating that Grindle was in the air for less than five minutes given there were no witnesses to the flight.

Grindle's close friend Adam Spicer said the man was a careful and judicious athlete who would have backed away from a launch had he had any concerns about the weather or terrain. He was an experienced skydiver, speed glider and "BASE jumper," an extreme sport that involves parachuting off of fixed objects such as buildings and cliffs.

"Walden was an expert in these sports," he said. "He certainly understood the risks; I don't think he was reckless."

State Parks spokeswoman Vicky Waters said Grindle did not have a permit to launch from the Robert Louis Stevenson Memorial State Park, which covers the 4,300-foot peak and most of the surrounding slopes. Only members of the Sonoma Wings Hang Gliding and Paragliding club are authorized to launch off the mountain under a long-standing special event permit, she said.

Grindle or other individuals could apply for such a permit separately from the club, she said, but there was no record he had ever done so.

A person found using the park in an unauthorized manner, she said, would be advised to leave and could be cited by a state park ranger, though there is no permanent ranger station at this park. Looking back over records from the past 10 years, she said, there had been no previous reports of unauthorized gliding or parachuting.

Grindle was not a member of the Sonoma Wings club, President Greg Sugg said. The special event permit, which must be renewed annually, requires that gliders be club members or be accompanied by a club member.

Club members said Mount St. Helena is not a particularly dangerous mountain to launch from, but it does require some technical ability since the nearest safe landing spots are far away on the floor of Knights Valley.

Spicer said his friend had launched from Mount St. Helena frequently. He dismissed talk of the permit requirement as part of turf war between the older hang glider and paraglider community and the newly emerging "speed glider" movement.

It's not clear exactly where Grindle launched, Stuart said, nor is it clear what caused the crash.

Speed gliders and paragliders employ similar fabric wings - essentially long rectangular parachutes that function like kites. The operator runs forward, allows the glider to fill with air, and then lifts off from a slope, rising into the air.

The speed glider wings are smaller, as little as half the square footage of a standard paraglider wing. That gives them a much higher speed and a lower descent angle, placing the operator closer to the ground and making for a shorter flight, said Reavis Sutphin-Gray, the paraglider representative on the Sonoma Wings' board.

The slopes of Mount St. Helena are steep and covered in brush and forest, "none of which is very inviting to put down a glider of any sort," he said. The area in which Grindle was reportedly found was not along the normal flight path club members would use.

Spicer said his friend Grindle was thoroughly familiar with the terrain and the possible landing areas on the slopes as part of his work as a manager of geographical information systems at Jack Neal & Son Vineyard Management. He spent most of his work day walking nearby vineyards and studying maps.

Speed gliding had become Grindle's main passion in recent years, Spicer said.

He said his friend had become somewhat more cautious in recent years, as he dated, and recently married, his wife Danielle. The couple had planned to move to the Lake Tahoe area, where they own a house, in part because of their shared love of skiing and other outdoor sports.

You can reach Staff Writer Sean Scully at 521-5313 or sean.scully@pressdemocrat.com. You can reach Staff Writer Derek Moore at 521-5336 or derek.moore@pressdemocrat.com.