PUTRAJAYA (THE STAR/ASIA NEWS NETWORK) - Thirteen individuals were involved in the hijacking of the MT Orkim Harmony, with five remaining at large, the Malaysian Maritime Enforcement Agency (MMEA) confirmed.

MMEA deputy director of operations Ahmad Puzi Ab Kahar said besides the eight that were arrested in Vietnam waters on Friday, there were another five individuals manning a tugboat used to approach MT Orkim Harmony.

The five individuals went separate ways in the tugboat, which was later found in Batam by the Indonesian navy.

"The five are currently still at large. In the meantime, we are using all means to ensure the eight are extradited to Malaysia so we can question them further," said Datuk Ahmad Puzi at a press conference here.

He also said further investigations by Vietnamese authorities revealed that the suspects were not amateurs, but "seasoned criminals".

"Looking at how they conducted the robbery, switching off the Automatic Identification System (AIS) so the could go undetected and how they were found with wads of cash on board, it showed that they knew what they were doing," he said.

He added that Malaysia was doing everything it could to ensure the suspects are extradited as soon as possible.

The MT Orkim Harmony, which was laden with 6,000 tonnes of petrol worth RM21 million (S$7.5 million), was reported missing at 8.57pm on June 11 while on its way to Kuantan Port from Malacca.

The vessel, owned by Magna Meridian Sdn Bhd, was carrying 22 crew members on board, comprising 16 Malaysians, five Indonesians and a Myanmar national.

The cargo belongs to Petronas.

Eight suspects were arrested in Vietnamese waters near Tho Chu Island last week, and confessed to hijacking the vessel.