He thought the "prank" was just to scare the full-time national serviceman, so once the group of officers put Corporal Kok Yuen Chin down next to the pump well, Staff Sergeant Muhammad Riduan Mohamed Sairi raised his arms and walked away.

"I raised my hands to signal I was stopping and said, 'I don't want to get involved'," said the Singapore Civil Defence Force serviceman, testifying yesterday during the ongoing trial where two high-ranking officers linked to Cpl Kok's death are contesting charges against them.

On the night of May 13 last year, Cpl Kok, 22, drowned after he was pushed into the Tuas View Fire Station pump well, also known as "kolam", in a ragging ritual that was part of activities to mark the completion of his national service.

The officers on trial - Kenneth Chong Chee Boon, 38, a lieutenant, and Nazhan Mohamed Nazi, 40, a first senior warrant officer - were rota commander and deputy rota commander, respectively, and were in charge of the station then.

They were each charged with aiding a rash act that caused grievous hurt by illegal omission. The two had allegedly failed to prevent a group of officers from making Cpl Kok enter the pump well, thereby endangering his life.

Yesterday, Staff Sgt Riduan said he had known before the incident that Cpl Kok could not swim.

That night, the officers on duty gathered for a cake-cutting ceremony for Cpl Kok at the fire station's control room. After it ended, Staff Sgt Riduan and two other officers lifted the NSF and carried him to the pump well.

"I thought we were going to place him at the kolam and scare him," he said, adding during cross-examination that he left the scene as he thought "everything was going to be over after we put him down".

In security footage of the incident, Staff Sgt Riduan is seen raising both arms and walking away from the pump well. He said this happened just as they heard Chong call out to them from the nearby control room to tell them not to record any video. Other officers were also seen raising their arms in the footage shown in court, and earlier witnesses had said this was done in response to Chong's instructions.

When asked what he thought could have happened to Cpl Kok at the well, Staff Sgt Riduan said the group could have splashed water at him or made him enter the well "voluntarily or involuntarily".

Staff Sgt Riduan said he headed to the second floor of the station, and found out something had happened only when he saw fire engines reversing in the yard. He ran downstairs to help in the rescue.

Nazhan's lawyer asked if he thought the act of carrying Cpl Kok to the well was ragging. Staff Sgt Riduan said he did not, adding no one intended to harm or injure the NSF.

The trial continues in September.