Firefighters return from the warehouse area, where they were involved in rescue operations.[ZHU XINGXIN/CHINA DAILY]

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

The dangerous chemicals stored in the warehouses that exploded on Wednesday night in Tianjin Port can not be determined at the moment, authorities said at a press conference on Friday.

Gao Huaiyou, deputy director of Tianjin's work safety watchdog, cited major discrepancies between the accounts of company management and customs, and damage to the company's office as reasons they are unable to identify the chemicals.

Cargo is stored in a warehouse for no more than 40 days before being transferred elsewhere, Gao said, adding that the blast sites have been redesigned to contain dangerous chemicals.

Hazardous substances have been detected from two drainage outlets near the blast site in Binhai New Area, the Ministry of Environmental Protection said on Friday morning, refuting rumors that the dangerous participles have polluted the surrounding area.

The harmful pollutants, including chemical Oxygen Demand (COD) and cyanide - a highly toxic chemical compound, were found three times and eight times higher than the usual levels, according to the ministry.

The Tianjin municipal environmental watchdog has set up 22 monitoring stations, of which five are checking water quality.

The drainage outlet to the Bohai Sea has been closed, and the bureau is conducting an investigation into water quality.

The blasts in the city of Tianjin on Wednesday night killed at least 55 people, including a dozen fire fighters. About 700 people were injured, 70 seriously.

Rescuers pulled one survivor from the wreckage on 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.

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

The explosions at the port, one of the world's largest, were so big they were seen by satellites in space and registered on earthquake sensors.

A team from the International Atomic Energy Agency's Beijing environmental emergency response centre, as well as 214 Chinese military nuclear and biochemical materials specialists, had gone to Tianjin, Xinhua said.

Several thousand residents were moved to 10 nearby schools after apartment buildings and homes were damaged, mainly by shockwaves from the explosions, it said.