Officers want to know where the Singapore dollars and Malaysian ringgit came from and whether the money was illegally obtained in the first place, said Pol Maj Gen Supisarn Bhakdinarinath, commander of the Crime Suppression Division.
Police could take action if they have evidence that the money was not clean, with no need to wait for a complaint to be lodged, he added.
The move follows the arrest of four men in Pattani and Songkhla on Thursday. They are Thawee Nitpran, 44, who is a former ship captain, Phan Sangthong, 45, Prasert Somchuay, 47 and Samphan Wannurat, 47. Police said they confessed to committing the crime.
They were among eight men sought on arrest warrants on charges of armed robbery and intent to kill.
Two more suspects - Kittipitch Thipkonglas, 42, and Somyos Sudluea, 43 - were arrested in Nonthaburi on Saturday.
Pol Col Thinnakorn Rungmart, chief of the CSD's division 6, said the two were captured in front of the Tharinporn Housing Estate in Bang Kruai district by CSD officers working with the Metropolitan Police Bureau. The two denied the involvement in the crime.
Mr Kittipitch had cash in ringgit worth the equivalent of 8 million baht when he was caught, and Mr Somyos had Malaysian currency worth 5 million baht.
Pol Maj Gen Supisarn said on Saturday that officers were going after the last two suspects, Kasem Khanpan and Arkhom Poonchana, and had established a separate investigation into the nature of the money.
The capture of the six pirates came after Tangthai Banmahing, a representative of the money changing firm Sahasup International, reported the robbery in Thai territorial waters to the CSD on Oct 9.
Ms Tangthai told police that on Oct 2 the company put 2.6 million Singapore dollars (around 65 million baht) and 5.6 million Malaysian ringgit (around 54.88 million baht) on board the boat, which was to deliver the cash to customers. There were a total of eight people on the vessel.
When the boat arrived in waters off Nong Chik district of Pattani en route to deliver money to a major client on Losin island, the company's radio centre was notified that a speedboat had approached the vessel with armed men on board who had opened fire. The crew reported they were under attack. When the company tried to call them back, there was no reply.
The pirates took all the money that was on board, according to Ms Tangthai.
Pol Maj Gen Supisarn said on Friday that investigators had checked currency exchange shops in southern provinces. They found out that banknotes with the company's mark on them been exchanged at one shop.
Police reviewed the closed-circuit camera footage from the shop and identified a suspect who was a former employee of the company who had been fired.
Investigators then determined that the pirate gang comprised a former captain and crewmen from the vessel who were all previously fired. They were angry and plotted the robbery as revenge, investigators said.
The group contacted a friend who was still working for the company and he became their accomplice.
When the pirates approached the boat and opened fire, the accomplice urged the other crew members to abandon the boat and then jumped into the sea. They followed him.
The pirates then made off with the money they knew would be on board, picked up their accomplice from the sea and killed the other seven crewmen, said Pol Maj Gen Supisarn.
They also later sank the company's boat and burned the speedboat they used to destroy the evidence. Police have not yet found the bodies of the missing seven crewmen but assume that they are dead.
A total of 53 million baht of the loot has been recovered. Police are interrogating the suspects and hope to get back more of the stolen money.