BEIJING (AP) — Safety inspectors say the operators of China's largest silver mine bought a broken down bus online, then overloaded it with miners, leading to brake failure and a crash that left 21 dead and 29 injured.

The head of the Emergency Management Ministry's department of safe production infrastructure, Pei Wentian, told state broadcaster CCTV the bus had not been inspected or registered and had a carrying capacity of 30 passengers. Pei said it was also operating in a mine tunnel on ramps not intended for transporting people.

The bus crashed Saturday morning into a protruding wall inside the tunnel at the mine operated by the Yinman Mining Co. in the sprawling Inner Mongolia region.

Executives of the company have been placed under travel restrictions while the investigation is underway.