Denver (AP) — Investigators were trying to figure out Friday what led an elevator to malfunction at a former Colorado gold mine, killing one person, injuring four others and leaving 12 people trapped for hours at the bottom of the tourist attraction 1,000 feet (305 meters) beneath the surface.

The elevator was descending into the Mollie Kathleen Gold Mine near the town of Cripple Creek in the mountains near Colorado Springs when it had a mechanical problem around 500 feet (152 meters) beneath the surface. That caused the death of one person, yet to be identified, and injured four others, Teller County Sheriff Jason Mikesell said in briefings Thursday night.

But what exactly caused the visitor’s death was not immediately provided. Eleven other people, including two children, who were riding the elevator were rescued. Four had minor injuries including back pain, neck pain and arm pain, the sheriff said.

Twelve adults from a second group were trapped for about six hours Thursday below ground. They had access to water and used radios to communicate with authorities, who told them there was an elevator issue, Mikesell said.

RELATED COVERAGE Trump heads to Colorado to drive his anti-immigration message 1 person dies and 12 are rescued after elevator malfunctions at Colorado gold mine tourist site Trump seizes on one block of a Colorado city to warn of migrant crime threat, even as crime dips

Mines that operate as tourist attractions in Colorado must designate someone to inspect the mines and the transportation systems daily, according to the state Division of Reclamation, Mining and Safety. Mikesell said he didn’t know the date of the last inspection. Records of the inspections weren’t immediately available online.

Engineers worked to make sure the elevator was working safely again before bringing the stranded visitors back up on it Thursday night. That included sending the elevator down empty to the bottom of the shaft to make sure it could get back up without issues. The elevator ride typically takes about two minutes, according to the mine’s website.

The 12 stranded tourists were hoisted up in groups of four over a half-hour period, the sheriff’s office said in a news release. They had been prepared to bring them up by rope if necessary, had the elevator not been usable.

The incident, which was reported to authorities at about noon, happened during the final week of the Mollie Kathleen Gold Mine season before it shuts down for the winter, Mikesell said.

Cripple Creek is a town of about 1,100 in the Rocky Mountains southwest of Colorado Springs.

The mine opened in the 1800s and closed in 1961, but still operates tours. Its website describes a one-hour tour. It says visitors can see veins of gold in the rock and ride an underground tram.

A woman named Mollie Kathleen Gortner discovered the site of the mine in 1891 when she saw quartz laced with gold, according to the company’s website.