A cave-in at a gold mine in neighboring Sonora, Mexico, killed three people Tuesday. The cause of the collapse has not yet been determined.

The three men were trapped when an earthen dome collapsed at the San Francisco mine near Santa Ana, Sonora, according to officials.

Rescue teams from the mine and local authorities recovered the bodies of the three men after hours of searching.

The mine is owned by the Canadian company Magna Gold. The company said in a statement that the cause of the cave-in is unclear but has promised to carry out an investigation, and to reinforce its safety protocols in the wake of the accident.

Sonora is considered the most important state for mining in Mexico, with a third of the country’s gold produced in the state in 2019. It has also been the site of major mining accidents, including a 2014 spill from Grupo Mexico's Buenavista copper mine considered the worst environmental disaster in Mexican history.

The San Francisco mine has been in operation since 2009.