The group was reportedly trying to reach Taiwan when mainland authorities picked them up on August 23.
Chinese authorities have formally approved the arrests of 12 Hong Kong activists caught last month while allegedly trying to flee the territory for Taiwan.
The group was picked up some 70 kilometres (43 miles) southeast of the city on August 23 while trying to escape by boat, authorities said at the time, adding that they were handed to police in Shenzhen, the southern mainland city bordering Hong Kong.
The 12 had since disappeared into China’s opaque judicial system, with lawyers struggling to access them and family members expressing fear over their fate.
On Wednesday the People’s Procuratorate of Yantian District in Shenzhen said it had approved the arrests.
Two of the detainees, referred to as Deng and Qiao respectively, were arrested on suspicion of helping the others escape Hong Kong.
These names were likely to refer to the Chinese surnames of detainees Tang Kai-yin and Quinn Moon.
The other 10 – including suspects with the names Li and Huang – were arrested for making illegal border crossings.
The case remains under investigation, the statement added.
Families of the 12 said in a statement they were “shocked and concerned” by the approval.
Hong Kong’s Security Bureau confirmed that mainland authorities informed local police of Wednesday’s approval, but declined to comment on families’ complaints of lawyers being barred from visiting the detainees.
Read more: Aljazeera