Hong Kong police on Thursday cracked an alleged money-laundering syndicate that is also suspected of funding protesters involved in the city's protracted anti-government protests.

Three males and a female, aged 17 to 50, were arrested, and about HK$70 million ($9 million) has been frozen after police traced suspicious cash transactions through a shell company in the past six months.

Police did not rule out the possibility that the money was used to encourage teenagers to join the violent protests, said Chan Wai-kei, acting senior superintendent of the Narcotics Bureau's financial investigation division.

The four suspects are believed to be linked to Spark Alliance HK — an online platform claiming to offer support for those detained in unlawful assemblies and protests. The platform had collected about HK$80 million in online donations in the past six months.

The alliance said the donations would be used to help those arrested to post bail, and for legal assistance and emergency medical fees.

However, Chan said most of the funds that it raised had been transferred to a bank account owned by a shell company and used to invest in personal-insurance products, which is not consistent with the company's business scope. The beneficiary is also in charge of the shell company.

During the operation, police also confiscated HK$130,000 in cash, six arrows, two laser pointers, receipts for supermarket coupons worth HK$160,000, and a considerable amount of gear such as helmets and gas masks.

Chan said police will continue to cooperate with the Department of Justice to trace the source and use of the funds.

Last month, HSBC reportedly closed a corporate account used to collect funds for the alliance after discovering that its transactions failed to comply with the business purposes stated by the client.

A former banker familiar with the matter told China Daily that although banks do monitor closely abnormal transactions at the request of the Hong Kong Monetary Authority, the supervision of accounts owned by fundraising civic groups is "not that tight".

Ida Xue contributed to this story.