BANGKOK (Reuters) - Three workers were killed in an explosion at an ethanol plant in Thailand's northeastern province of Khon Kaen, police said on Monday.

The three workers were repairing the lid of a wastewater tank at an ethanol plant which belongs to Thai Ethanol Power Plc in the province of Khon Kaen, 390 km (240 miles) from the capital Bangkok, said Police Lieutenant Colonel Sarawut Dansuk.

"It could be that there was some kind of gas that leaked out and caused sparks when they were welding the tank lid and that set off an explosion," he said.

"An autopsy showed they were killed by the impact from the explosion."

The three killed were Thai nationals, he added.

Police are investigating whether the ethanol company was responsible for the accident.

(Reporting by Panarat Thepgumpanat; Writing by Patpicha Tanakasempipat; Editing by Amy Sawitta Lefevre and Christian Schmollinger)