Thai police say they expect to issue more arrests in an alleged conspiracy to exploit the name of Crown Prince Vajiralongkorn for personal gain.
Last week three men, including a famous fortune-teller, were charged with lese majeste, or damaging the monarchy.
One of them, a prominent policeman, died in custody on Friday. The police say he hanged himself using his shirt.
Thai authorities have publicised this case, in contrast to previous trials under the draconian lese majeste laws.
On Wednesday, the national police chief, Chakthip Chaijinda, presented reporters with an assortment of items - including expensive guitars, guns, watches and Buddhist amulets - that were described as evidence against the fortune-teller.
Until his arrest last week, the fortune-teller had been seen frequently with top officials. He recently played a prominent role in promoting the image of Crown Prince Vajiralongkorn, heir to the throne.
The police say he exploited that position to demand money from some of Thailand's biggest companies - but details are very sketchy.
Nor could they clearly explain how the arrested police officer ended up dying in custody. They refused an independent autopsy.
This case has striking parallels with a purge of the Crown Prince's estranged wife and her family last year. They too were accused of abusing his name to make money.
The key police officer in that case also died in mysterious circumstances.
The lese majeste law bars journalists from saying anything negative about the royal family.
However, it is no secret that the extremely frail health of King Bhumibol, who has been on the throne for 69 years, is a source of acute anxiety in Thailand.