Pol Lt-General Sanit Mahatavorn said he would take all things into consideration and base the probe on justice and fairness.

He was referring to widespread criticism that police might side with CPF in the investigation because it is a big company.

On Friday, a Chulalongkorn University student, on her third day of an educational visit to the CPF plant in Bang Na district, died after falling into a wastewater pond. Four CPF employees who tried to save her also died.

Hydrogen sulphide, a poisonous colourless gas, may have contributed to their deaths. But questions remain about the cause of the incident.

Sanit said police were collecting evidence which they would compile with experts’ opinions to learn what actually happened. He declined to disclose information police obtained from interviews with witnesses, saying police needed evidence to back up any claims.

“We are looking into the details to establish who should be responsible for the criminal charge and civil charge in which employers have joint responsibilities with employees for damage to outsiders,” Sanit said.

Because of a lack of CCTV cameras at the scene of the accident, police have to be thorough in their investigation, he said.

Sanit also referred to the surrender of Preecha Tamporn, 51, the company’s chief of wastewater ponds. Preecha turned himself in about 4pm on Monday at Bang Na police station before being questioned for about four hours.

He was then charged with negligence, causing deaths, before being released on bail. The penalties are a jail term of not more than 10 years and a fine not exceeding Bt200,000.

Sanit said Preecha was at the area when the tragic accident happened. “I assigned police to check the factory plan to know every details of the crime,” he said.

The police chief added that the accident sent a signal to operators and factories to examine risk areas ensure maximum safety for everyone.