The captain and a crew member of the boat involved in the worst tourism accident in Vietnam on Thursday were arrested by local police on Sunday.
The accident happened when a boat with sleeping tourists sank in the early hours of the monring in Ha Long Bay, killing 12 tourists.
Colonel Vu Chi Thuc, head of the northern Quang Ninh province's police department, said captain Nguyen Van Minh, 22, and Do Van Thang, 27, were being investigated for "lacking responsibility causing serious consequences."
Thang was in charge of the engine maintenance on the boat.
On Sunday morning, the Truong Hai 06 boat was towed to the Ba Lan shipyard area for the police to conduct further examination.
The Associated Press quoted Quang Ninh provincial police spokesman Le Thanh Binh as saying on Sunday that initial investigations show human error was to blame for the accident.
The man in charge of engine maintenance forgot to close the valve that allows water to come into the boat to cool the engine before he, the captain, and other crew went to bed.
The boat owner, Truong Hai Co, was involved in another deadly accident in September 2009 when it operated under a different name.
In the previous tragedy a boat of the company's sank during heavy rain, killing three foreign tourists and a local guide.
Meanwhile, the bodies of eight out of ten foreigners who died in the Ha Long Bay tragedy have been released to their relatives and embassies as of Saturday afternoon, local reports said.
Ha Long Bay is a UNESCO World Heritage site known for its stunning limestone cliffs in a large body of backwaters, about 160 kilometers east of the capital Hanoi. It attracted more than two million visitors last year.