CHONGQING -- Rescuers are working around the clock, searching for 20 miners who remain trapped in a colliery in southwest China's Chongqing Municipality nearly 24 hours after a gas explosion.

Altogether 35 workers were in the pit when the accident happened. Two escaped unharmed leaving 33 trapped in a pit at Jinshangou Coal Mine, Yongchuan District at 11:33 a.m. Monday.

The police are looking into possible misconduct by the mine's management team. Jiang Wenge, legal representative of colliery, and the manager of the mine are being questioned, according to Luo Qingquan, head of the district government.

As of midnight, rescuers had retrieved 13 bodies, said Chongqing's deputy mayor Mu Huaping at a press briefing in the early hours of Tuesday.

The official corrected previous casualty reports provided on Monday, which said 15 miners were confirmed dead. "We are still going all-out in our search for the 20 missing miners," he said, "and will continue to do so as long as there's still a ray of hope."

More than 400 rescuers worked through the night, digging carefully through debris from the collapse to avoid secondary disasters. At least 20 ambulances and dozens of medical workers are standing by.

Gas has been cleared from one tunnel and work to clear another tunnel is expected to start on Tuesday afternoon.

When the explosion occurred, Tao Puzhang, 60, working on his land about 500 meters from the mine heard a bang and felt the ground shake under his feet.

Liu Fuxiu, a winch operator with the mine, working 40 meters from the entrance to the pit, was knocked off her feet by the blast and received injuries to her face and back. "My injuries are nothing," she said while responding to investigators. "I have a relative down the pit and I hope he is still alive."

Following the explosion, Chongqing has ordered a safety overhaul and temporarily closed all collieries with less than 90,000 tonnes of annual output.

The Jinshangou mine has a designed annual output of 60,000 tonnes, according to its license.