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An injured horse rider in Novato is lifted to an ambulance Thursday by Henry 1, the helicopter of the Sonoma County Sheriff's Office. The rider, Lisa Zeppegno, was thrown from her horse in open space off Indian Valley Road.

Marin authorities are trying to identify two young mountain bikers who illegally whipped down a rural Novato trail, causing two Oakland women to be thrown from their horses and injuring a horse that fled in a panic.

Lisa Zeppegno, who suffered spinal injuries in the incident, said the mountain bikers ignored their pleas for help and rode off. She was airlifted from the remote area some two hours later.

"I could've been killed," said Zeppegno, 44, of Oakland. "They came so fast and out of nowhere that the horses were spooked. I'm just lucky the horse didn't run over me."

The incident left the county's park director seething -- and vowing to hold the bicyclists and their parents accountable.

"Why aren't these kids being taught more responsibility?" said Linda Dahl, director and general manager of Marin County Parks. "Clearly they don't care. And I'm here to tell you, they're going to start caring."

The incident was reported at about 4 p.m. Thursday in the Indian Tree Preserve near the end of Vineyard Road, where Zeppegno and a friend, Nicole Devito, a 47-year-old Oakland resident, were riding slowly on a narrow trail. The women sponsor horses at Willow Tree Stables.

Zeppegno said the mountain bikers came around a blind corner, terrifying the animals. Both women were bucked off their horses, and Zeppegno's horse, Coco, ran away.

Zeppegno said the cyclists, who appeared to be 12 to 14 years old, did not stick around.

"Nicole was screaming at them and begging them to stay because we needed help, and they just left," she said.

Devito, who was not injured as badly as her friend, was able to get reception on her cellphone and called for assistance.

Novato paramedics responded on a fire road, but the area was so remote they had to climb down more than a mile to reach the women, said Novato fire Capt. Dmitri Menzel.

The fire department called for a Sonoma County Sheriff's Office helicopter. The flight crew sent a medic down on a long line to attach Zeppegno to a stretcher.

Zeppegno was moved to an ambulance and taken to Kaiser Permanente Medical Center in Santa Rosa. She learned she had compression fractures of her L1 and L2 lumbar vertebrae and her sacrum, the bone at the base of her spine.

Zeppegno, who was released Friday afternoon, said it could take six to eight weeks to have normal mobility. She is an acupuncturist.

"I'm going to lose a lot of income, and I'm worried about that," she said.

Devito suffered neck pain and minor injuries but did not require hospitalization.

The disappearance of Coco the horse, an 8-year-old Belgian draft quarter horse cross, set off an extensive search, including members of the county Open Space District, Marin Search and Rescue and the sheriff's volunteer mounted posse.

Also assisting the search was Tom Boss of the Marin County Bicycle Coalition. He said the organization has been trying to educate mountain bikers about the trail rules.

"The majority of the mountain bikers are riding trails that they're supposed to be on," said Boss, a Forest Knolls resident. "You get an incident like this, it paints all the users in a bad light."

Coco remained missing until Friday afternoon, when she was found by Ian McLorg, a seasonal ranger. The horse suffered from deep gashes on her nose and in her armpit, said Monte Kruger, co-owner of the horse and Willow Tree Stables.

"She's going to require pretty extensive veterinary care," said Kruger, who runs the stable with her husband, Curt. "She's in shock. Her flanks are quivering. Her respiration is up. She's dehydrated. She's been in a gully for 24 hours with no food or water."

County authorities are working to identify the illegal mountain bikers who fled the scene. The Krugers urged them and their parents to come forward.

"There's humongous issues here and serious damages and consequences," Curt Kruger said. "Very often the parents protect the kids -- 'Oh, they're just playing out there.'"

The Open Space District's website says bikes are banned on district lands "except upon fire protection roads, designated bicycle pathways or public roads not signed against such use."

Monte Kruger said she sees illegal bike riders about twice a week.

Dahl, the parks director, said she has heard young mountain bikers openly brag at public meetings about riding on illegal trails, saying they considered the fines to be "user fees." Fines and penalties for illegal biking in open space range from $199 for the first offense to $615 for the third and subsequent offenses.

She did not have statistics on the number of reported incidents, but said Indian Valley and Mount Burdell are becoming increasing problem areas.

"It's dangerous for the animals, it's dangerous for the people, and it's very disturbing to us," she said. "We're going to follow through. We're going to have a community conversation about this."