Chinese authorities say a massive coal mine blast that killed 148 people last month was caused by the operators' failure to realize that extending the mine's shaft would greatly increase its gas level, state media reported Sunday.
Insufficient ventilation in the mine allowed the gas to build up and mix with fresh air, and sparks from mine machinery ignited the explosion, government-run newspapers said.
The Oct. 20 explosion at the Daping Mine in Henan province was the country's deadliest mine accident since 2000.
It occurred "deep in a complicated stratum" inside the mine, the Xinhua News Agency said. When mine operators extended the shaft, gas rushed out.