The crewman in charge of the boat’s engine went to bed and forgot to close the valves that allow water to flow in to cool the motor, said Quang Ninh provincial police spokesman Le Thanh Binh. A pipe that carries that water ruptured, allowing water to leak into the boat while the passengers slept.

The boat’s captain Nguyen Van Minh, 22, and engine man Do Van Thang, 27, have been arrested and accused of “negligence causing serious consequences,” which carries a jail term of up to 12 years.

Eleven foreign tourists from nine countries died along with a Vietnamese tour guide when the “Dream Voyage” suddenly sank around 5 a.m. on Thursday. Most tourists were still sleeping and became trapped inside their overnight cabins when a torrent of water quickly inundated the boat and sucked it under.

The valves are supposed to be closed when the boat is anchored. However, leaving such valves open should not by itself cause the vessel to fill with water, Binh said.

The bodies of 11 vacationers from the U.S., Britain, Australia, Japan, Russia, France, Sweden and Switzerland have all been handed over to families or embassies. The remains of their Vietnamese tour guide have also been released.

Online Dan Tri newspaper reported Monday that the boat’s insurance company will pay up to 20 million dong ($950) to each of the victims’ relatives, having already compensated two families.

It quoted SHB-Vinacomin executive Nguyen Ngoc Lan as saying that insurance was included in the cost of the boat ticket. Quang Ninh provincial officials are also providing an additional 20 million dong ($950) to the victims’ families.

The incident marked Vietnam’s deadliest tour boat accident since opening up to foreign tourists 25 years ago. Local media have reported that the boat’s owner, Truong Hai Co., had a checkered safety record. While operating under another name in September 2009, another one of its tour boats sank in heavy rains and wind, killing three foreign tourists and two Vietnamese.

Ha Long Bay is a world heritage site and one of Vietnam’s must-see tourist destinations. It is located about three hours east of Hanoi in the Gulf of Tonkin near the northern Chinese border. The emerald water is dotted with 1,600 dramatic limestone formations that form tiny islands.

Some 500 boats are licensed to take tourists on cruises. Many opt for day tours, while others prefer to sleep in overnight cabins on wooden junks, ranging from budget to luxury accommodation.

Officials have said all boats plying Ha Long Bay will undergo fresh safety inspections in addition to the annual checks.