The tanker, which belongs to a shipbuilding company in the northern port city of Hai Phong, was carrying a crew of 18 people and over 5,000 tonnes of oil when it vanished from radar 40 minutes after leaving Singapore on October 2 for Quang Tri province in central Vietnam.

A week later, the ship and its crew were released and arrived in Quang Tri on October 9.

The result of black-box testing is impending, said chief of the Police Department for Investigation of Social Order-related Crimes, Maj. Gen Ho Sy Tien on October 23.

The department will work with Interpol and a Singapore-based centre for Southeast Asia piracy prevention to hunt down attackers, whose nationalities remain unknown.

Once sufficient evidence is collected, the police will take legal proceedings in accordance with Vietnamese law.

The Sunrise was the 12th such piracy case since April in Southeast Asia where tankers have been hijacked and then released after the cargoes were taken off, according to the International Maritime Bureau.

Vietnam is now a member of the Southeast Asia and Asia conventions on countering piracy.