CHLORIDE, Ariz. -- A 10-year-old Arizona girl remained in serious condition Tuesday at a Las Vegas hospital after being injured when an all-terrain vehicle plunged into a 125-foot mine shaft, killing her 13-year-old sister.

Casie Hicks remained in the University Medical Center Pediatric Intensive Care Unit. UMC spokeswoman Cheryl Persinger said no other information on Casie was available.

The accident in an unmarked shaft left Casie with "major injuries" and killed her sister, Rikki Howard.

No signs or barriers were at the site of the inactive mine in Chloride, about 17 miles north of Kingman, Mohave County Sheriff's Department spokeswoman Sandy Edwards said.

"They were driving along and they went into the mine. It was a total accident," Edwards said.

The girls were on the three-wheel ATV riding with their father, who was on a dirt bike, around 7 p.m. Saturday. The girls' father said he was riding ahead and noticed the girls were missing.

He told the sheriff's office he went back to look for the girls, but did not find them.

Edwards said sheriff's personnel tracked the ATV to the mine shaft at 6:20 a.m. Sunday. The shaft is on the edge of a dirt road hidden by a screen of bushes.

"Suddenly the father called out and received an answer from one of the girls from off the road," said Capt. Greg Smith of the sheriff's office.

Crews rappelled into the mine, and found the girls and the vehicle at the bottom.

Howard was pronounced dead at the scene and was pulled out of the mine shaft around 10:40 a.m.

Hicks was airlifted to UMC in Las Vegas.

"A 10-year-old girl has spent the night at the bottom of a mine shaft, which is the most horrifying thing I can think of," Smith said.

The girl's family said through hospital spokeswoman Cheryl Persinger that they didn't want to talk to the media.

Neighbor: Girls 'Were Loved By Everybody'

Seth Johnson, a neighbor of the girls and their family's landlord, said the two were half-sisters.

Johnson said the girls' family was at the Las Vegas hospital. "It's an awful shock," Johnson said. "Their parents are very distraught."

Johnson said Rikki resembled and liked to act like Paris Hilton. "She was pretty concerned with makeup and looking good," he said.

Casie, he said, loved to play the violin.

He said his stepson often went riding with the two girls on dirt bikes and ATVs.

He said he helped search for the girls Saturday night and thought they may have just gone off the road and would be found in the morning.

Cathy Kelso, a bus driver for Mt. Tipton School in Dolan Springs, said she has been driving the two girls to school for a year and a half.

"They were loved by everybody," she said. "They're little sweetheart girls. I just keep hoping it's not true, but it's horrible."

Mines A Known Hazard In Chloride

Chloride is a place where all-terrain vehicles can be as common as cars and where desert hillsides are playgrounds, so residents were stunned but not surprised when the two young girls drove their ATV into the mine shaft.

Residents estimate there are dozens of abandoned mines in the hills surrounding the rural northwestern Arizona community, and they say that anyone who is going riding among them has to respect the terrain.

"They really are dangerous hills out there," said Bill Keller, who has lived in Chloride for 10 years and goes ATV riding in the hills regularly. "I don't go anywhere off a trail here. I am totally amazed that it hasn't happened before."

At the mine, the family members erected a cross that said: "In loving memory: Rikki." They placed yellow daisies on the cross and laid purple and orange tulips and a pink teddy bear at its base.

Brush next to the sheer-walled mine blocked any view of the shaft from an adjacent dirt road that was more like a trail.

"It's like a serpent's mouth waiting to swallow you up," Johnson said.

Johnson said earlier Monday that riding ATVs in the countryside was a common activity for the family. The hills, visible from the family's singlewide trailer, also are popular with other ATV riders.

Some residents say it's also not uncommon to see children on ATVs.

"These mountains are everyone's backyard," said Russell Agee, who at least once a week goes up to the hills where the accident happened. "The trails are a lot of fun if you're careful."

He said there are some places where there are mine shafts next to the road. "A lot of them you can't tell they're there until you're right on them," said Agee, who was riding through town Monday on a mud-spattered four-wheel ATV.

The ownership of the shaft where the girls fell had not been determined, according to the Arizona State Mine Inspector's office.

Officials say there are an estimated 100,000 abandoned mines in Arizona, not uncommon in a state with a long history of gold and silver prospecting and other kinds of mining activity. Many date back to the late 1800s and early 1900s, and sometimes it's not possible to trace ownership.

Many were in remote areas, but sprawling development has brought people closer to them. State Mine Inspector Joe Hart said that ATVs and dirt bikes also now provide access to areas that previously were largely unreachable.

"You can go anywhere and everybody's curiosity is raging when they're on those things," Hart said.

Community members have begun collecting money for the family.

"What a shame for children so young," said Bonnie McNeely, the owner of Chloride's only restaurant, Yesterdays. "How quickly tragedy can happen. My sympathy goes out to the family."

Abandoned Mines Prevalent In Arizona

Laurie Swartzbaugh, deputy director of the Arizona State Mine Inspector's office, said that since Jan. 1, the office has secured 108 of the roughly 100,000 abandoned mines scattered around the state.

"There's a significant amount of abandoned mines out there that are hazardous to the public's health," she said. "Most of those mines are from old prospectors who would go in and they would mine and they'd just pick up leave. And there are some very dangerous ones that are very deep."

She said abandoned mines can be up to 40 feet deep or more.

In the Phoenix area, there are more than 12,000 abandoned mines. Officials said the mines are a big problem when considering the Valley's growth. Experts said the abandoned mine shafts can be extremely dangerous, especially if children are playing nearby.

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