Sgt-Major Sanchai and Longma Limwilai weep while claiming the body of their son Natthaphongchai from a police morgue yesterday. The 17-year-old student died while practising an underwater welding drill.

Pol Major Surajit Plienprasert, the senior case investigator, said police would wait for the autopsy results before continuing their inquiry into the tragedy.

He said the coroner's initial report suggested the three students drowned because they ran out of oxygen, or succumbed to gas poisoning after acetylene or LPG used in welding leaked into the oxygen conveyer system.

Surajit said a school instructor told him the three students, who had been given a lecture about underwater welding in the morning, asked for a key to a changing room.

He said they might have secretly taken keys to the water tank room during the lunch break and dived on their own without supervision.

"Police do not have much information at hand because the school staff has not provided us with the information that they are supposed to," he said.

Phaholyothin police and MTTS school staff had earlier given contradictory information about the incident, which happened at about noon on Tuesday.

One police officer said a fourth student pulled the others out of the five-metre-deep water tank and they vomited heavily before passing out.

But a school lecturer said the three students were already dead when he pulled them from the tank by grabbing their individual oxygen hoses.

The parents of the three students criticised MTTS staff for the delay in notifying them about the deaths of their children and for withholding information they said they were entitled to know.

One father was threatening to file lawsuits against the school if the autopsy results contradicted initial details given to them by the school.

Prasert Chattrakool, father of Kawin, said a relative who received a call from the school was told his son and two other friends had "stolen" keys to the water tank room and dived on their own. He said the call was made at 4pm although the incident took place at about noon.

Prasert said he would wait for the autopsy results and would file a lawsuit against the school if details vary from what he was told. "I want the school to show responsibility. Don't just make the dead kids villains. Dead people cannot say or argue anything."

Army Sgt-Major Sanchai Limwilai, father of Natthaphong-chai, said his wife received a call from school employee at 4.30pm. The caller said his son had a road accident, and nothing else.

He said an Army major, who he thought was a school lecturer, told his son's classmates to tell him that his son drowned. But the major mentioned nothing about the comment by a classmate that he used CPR on Natthaphongchai before he died.