The death toll from the blast Tuesday evening at the mine in Henan province’s Mianchi County doubled after 13 more bodies were found, said an official surnamed Li from the Henan Administration of Coal Safety.

Another 20 miners managed to escape from the mine, he said, adding it was unclear why the mine had initially reported that only 33 miners were working underground at the time of the blast.

A police investigation is under way, the official Xinhua News Agency reported. Most such accidents are usually blamed on a buildup of coal gas or methane ignited by sparks or open flames.

The mine was in the process of being merged into the state-owned Yi Ma Coal Industry Group, Xinhua said.

It had been closed for technical upgrades but mine owner Suo Yonggang ordered workers back into the shafts, Xinhua reported, citing Li Guoqi, deputy chief engineer at Yi Ma.

It said Suo had fled and police were looking for him.

All the bodies have been recovered, Xinhua said.

With an annual production of 150,000 tons of coal, the mine is typical of the mid-size operations that China has been seeking to consolidate into safer, more efficient operations.

In a separate accident Tuesday, a coal mine tunnel in northeastern Liaoning province collapsed, leaving four miners dead, Xinhua said, citing local authorities. Three other men managed to escape from the shaft owned by the Caitun Coal Mine in Benxi City.

The state-controlled coal mine was under renovation when the accident happened. The provincial coal mine safety department set up a working group to investigate the accident.

China’s coal mines are the world’s deadliest, with thousands of miners killed every year. Massive demand for coal induces many producers to cut corners and sidestep regulations, although authorities have been able to reduce fatalities considerably through improved safety and the closure of small, often unlicensed mines.