Pol Colonel Krissana Pattanacharoen, deputy police spokesman, said police have still interviewed witnesses and gathering evidences.

“As of now, we have interviewed 10 witnesses including a friend of the student who was killed in the incident. The interviews were fruitful,” he said, adding he could not reveal the details of the interviews.

Two students were visiting the poultry food-processing facility to study its wastewater treatment system when one of them fell into the pipe system, CPF said.

CPF environment official Chatree Srisankhon, who was assigned to show the students around, jumped in to try and save the girl and three other workers also went in to help in a vain rescue bid.

Earlier, Parisothat Punnabhum, CPF senior executive vice president for human resources, said the wastewater-treatment pipe system was a controlled zone and barred to unauthorised persons as it could yield a gas causing unconsciousness.