Improper desulfurizer injections into the oil pipeline that exploded in the northeast China port city of Dalian on July 16 caused its explosion, results of a State Administration of Work Safety and Ministry of Public Security investigation showed Friday.

The 0.9-meter-diameter oil pipeline exploded at 6 p.m., triggering a smaller adjacent pipeline to also explode.

The explosion happened as workers from Shanghai-based Q.PRO Inspection and Technical Service Co. continued to inject desulfurizer into the pipeline after the 300,000-tonne tanker had finished unloading its oil at 1:00 p.m.

Produced by Tianjin-based Huishengda Petroleum Technology Co., the desulfurizer was strongly oxidizing, the investigation's report said.

A subsidiary of China National Petroleum Corporation (CNPC) had authorized the two companies to conduct the operation.

The State Administration of Work Safety's and the Ministry of Public Security's investigation accused the involved parties of failing to scientifically verify the safety of the oxidizer.

The report also said the involved parties failed to use standard injection procedures and identify risks promptly.

Lax management and ineffective oversight were also blamed for the accident as relevant departments failed to stop the injection operation in time.

The report also noted that a concentration of large tanks storing hazardous chemical added to risks.

The pipelines' explosion caused a blaze that took 15 hours to extinguish but there were no casualties.

The resulting oil spill "seriously polluted" 11 square-kilometers of sea and "slightly affected" 50 square-kilometers of sea, the local government said.

On Tuesday, authorities lifted a partial ban on maritime traffic at the port.

CNPC said Thursday the pipeline which exploded had been repaired and resumed normal operations.

The State Administration of Work Safety and the Ministry of Public Security Friday urged Chinese ports to ramp up safety measures ahead of the loading, unloading and transporting of oil and other hazardous chemical products.

The State Council, China's Cabinet, issued a circular on Friday, urging local governments and companies to strengthen work safety with enhanced supervision.

The accident is still under investigation.