VISAKHAPATNAM (India) • A clogged cooling system was the likely cause of a temperature surge in a storage tank which led to a gas leak at a chemical plant in India, killing 12 people, three state government investigators told Reuters.

They said workers at LG Polymers, a unit of South Korea's LG Chem near the southern city of Visakhapatnam in Andhra Pradesh, did not use a warning system to alert residents but informed company management after the leak in the early hours of May 7.

The three investigators asked not to be named as they are not authorised to publicly discuss the investigation, launched by the government of Andhra Pradesh state.

LG Polymers said the situation at the plant, which was in the process of re-opening after a weeks-long nationwide lockdown, was now under control.

About 800 people were taken to hospital after the leak, and some were asleep when gas drifted over their homes. Police have filed a culpable homicide complaint against LG Polymers.

When a Reuters witness visited the Venkatapuram village adjoining the plant on Tuesday, leaves on trees exposed to the gas looked shrivelled and had turned brown.

The three investigators said the temperature inside the oldest of the three storage tanks holding styrene monomer, a raw material used in polystyrene products, rose to over 150 deg C, more than six times the permitted level.

"The chiller unit was clogged because of a polymerisation process, which occurs when temperature exceeds certain levels," one of the officials said.

To slow the polymerisation process, a chemical reaction that generates heat, styrene monomer must be stored at 20-30 deg C with a cooling system and chemical compounds, or inhibitors, typically used to keep temperatures under control.

If unchecked, the heat generated from the polymerisation process causes the chemical to vaporise. But the cooling system was clogged and the likely cause of the blockage was a chemical reaction inside the tank holding the styrene monomer, the three investigators said.

The skeletal workforce at the facility overnight was unable to bring the situation under control, the investigators said. "When temperatures started rising, the staff at the plant were not able to use the cooling system," one investigator said.

"The team of production, environment and safety experts are currently investigating the cause of the incident," LG Polymers said in a statement on Thursday. One of the investigators said the gas leak was a result of a lack of safety precautions. "It was clearly a crime of omission," he said.

REUTERS