Investigators searched for clues Friday to identify what caused two huge explosions at a warehouse storing volatile chemicals at a busy port in northeast China

Investigators searched for clues Friday to identify what caused two huge explosions at a warehouse storing volatile chemicals at a busy port in northeast China, as foreign and local companies assessed the damage to their operations.

The blasts in the city of Tianjin Wednesday night killed at least 50 people, including a dozen fire fighters, state media said. About 700 people were injured, 71 seriously.

Rescuers pulled one survivor from the wreckage Friday, a city official told reporters. Columns of smoke from fires still burning rose from the blast site amid the devastation of crumpled shipping containers, thousands of torched cars and port buildings reduced to burnt-out shells.

The warehouse, designed to house dangerous and toxic chemicals, was storing mainly ammonium nitrate, potassium nitrate and calcium carbide at the time of the blasts, according to police.

Chemical safety experts said calcium carbide reacts with water to create acetylene, a highly explosive gas. An explosion could be caused if fire fighters sprayed the calcium carbide with water, they said.

The official Xinhua news agency has said several containers in the warehouse had caught fire before the blasts.

Lei Jinde, the deputy propaganda department head of China's fire department, a part of the Ministry of Public Security, told state-backed news website ThePaper.cn that the first group of fire fighters on the scene had used water.

"We knew there was calcium carbide inside but we didn't know whether it had already exploded," he said.

"At that point no one knew, it wasn't that the fire fighters were stupid," Lei said, adding that it was a large warehouse and they didn't know the exact location of the calcium carbide.

Xinhua has reported 18 firefighters remain missing, with 66 among the hundreds of people hospitalised.