Phoenix ? A 13-year-old girl who fell into a brush-covered mine shaft while riding an all-terrain vehicle was found dead at the bottom Sunday, and her 10-year-old sister was rescued with serious injuries, authorities said.

The girls, 13-year-old Rikki Howard and 10-year-old Casie Hicks, were out for a holiday weekend ride around 7 p.m. Saturday when their father, who was riding ahead of them on a dirt bike, noticed the girls were missing.

“They were driving along and they went into the mine. It was a total accident,” Mohave County Sheriff’s Department spokeswoman Sandy Edwards said.

The mine, believed to be inactive, is located in Chloride, about 17 miles north of Kingman. It was next to a dirt road, concealed by brush and had no signs or barriers.

Sheriff’s personnel searched throughout the night, but they weren’t able to follow the ATV’s tracks into the 125-foot mine shaft until 6:20 a.m. Sunday. The team walked by the site overnight because the hole was covered, she said.

When the entrance was discovered, the father called out and one of the girls answered, officials said. Crews later rappelled into the mine and found the girls and the vehicle at the bottom.

The 10-year-old girl was transported to University Medical Center in Las Vegas, Edwards said. She was in critical condition Sunday afternoon, a hospital spokesman said. The family declined to comment through the hospital.

Seth Johnson, a neighbor of the girls and their family’s landlord, said the two were half-sisters. Their family was at the Las Vegas hospital, he said.

“It’s an awful shock,” Johnson said. “Their parents are very distraught.”

Cathy Kelso, a bus driver, said she has been driving the two girls to school for a year and a half. “They’re little sweetheart girls,” she said. “I just keep hoping it’s not true, but it’s horrible.”

Laurie Swartzbaugh, deputy director of the Arizona State Mine Inspector’s office, said that the mine had not been used for some time, and that the office was investigating. She said abandoned mines are common in the state, and that since Jan. 1, the office has secured 108 of them.

“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.”

It was not immediately clear who owned the mine. Swartzbaugh said many abandoned mines date back to the early 1900s and sometimes it’s not possible to track down who owns them.