The blasts happened on Tuesday as some of the 25 crewmen were preparing for refuelling operations on board the 144-metre Aulac Fortune – a Vietnamese-registered tanker – off Lamma Island.

Photographs of the search operation released by the fire department showed a damaged cargo hold filled with murky water, twisted metal and other debris.

Initial investigation showed at least three of the seven 12-metre-high cargo holds were badly damaged by the explosions which also ripped open part of the main deck, according to a source. The access links beneath the main deck were also wrecked.

Members of the high-angle rescue team had to lower the divers into the cargo holds for the search, according to the department.

“Some of the cargo holds are filled with three metres of water. Due to the impact of oil pollution, there is zero visibility at the bottom of the holds,” the department said.

It said the greasy and slippery floor, the listing wreck and damaged structures hampered efforts and posed a danger to the rescuers.

Hung Shing-yau, senior station officer of the department’s diving unit, said water in the hold was mixed with oil and there were sharp metal fragments.

“Divers worked in zero visibility in the water and had to search by hand,” he said.

Yiu Men-yeung, the department’s divisional commander of marine and diving, said that as strong winds and currents hit the wreck, sharp fragments moved in the water and added to the danger.

He said initial investigation showed the man, 32, killed in the explosions and the two missing sailors were working on the main deck above the No 4 cargo hold.

“After the blasts, the deck area was ripped open, it is possible the two missing crew fell into the hold,” Yiu said.

He said the divers’ search of the hold on Wednesday night was fruitless.

As an assessment showed the tanker was stable and there was no risk of it sinking, firefighters boarded the vessel to search the superstructure, including the wheelhouse, on Wednesday morning.

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Divers were deployed at 6.30pm that day to go beneath the main deck to search three cargo holds. The operation was suspended at about 9pm and resumed on Thursday.

According to the department, they would search the remaining four holds, the engine room and other parts of the vessel.

The tanker was on its way to Thailand after unloading a cargo of oil in Dongguan, Guangdong province, but stopped near Lamma to refuel. It arrived in Hong Kong at 3am on Tuesday and anchored one nautical mile south of the island.

The incident happened at about 11.30am on Tuesday as the crew were connecting hosepipes on deck in preparation for refuelling. A barge carrying fuel was berthed next to the tanker, with four crew on board.

Local authorities said the refuelling had not started when the crew heard three explosions. After the blasts, the tanker caught fire. Firefighters took about five hours to douse the flames.

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The crew of the oil tanker jumped into the sea to escape or were hurled into the water by the force of the explosions. More than 20 crew members were plucked from the sea and brought on board two marine police launches.

The 32-year-old was killed and three others were injured in the blasts. The four crew on the oil barge were slightly hurt.

Marine police detectives were to compile a death report and study whether the event was an accident or if criminal intent was involved.

The Marine Department held a meeting on Wednesday with the relevant parties including the shipowner and shipping agent to assess the risk and prepare a salvage plan.