On the night he made a failed attempt to flee to Taiwan by speedboat, 16-year-old Hong Konger Hoang received a phone call, signalling it was time to leave at once.

The teenager is the youngest of a dozen Hong Kong activists now in mainland Chinese custody after trying to escape prosecution over last year's huge democracy protests, their boat captured by the Chinese coastguard.

They have since disappeared into the mainland's opaque and party-controlled justice system, compounding fears in Hong Kong that Beijing's authoritarianism is creeping into the finance hub.

Hoang was facing a charge of attempted arson for allegedly throwing petrol bombs. AFP is withholding his full name because of his age.

He has a troubled relationship with his family and was taken in by a network of politically sympathetic locals while he awaited trial in Hong Kong.

"When we first met, he barely talked," recalled Diana, who housed Hoang for three months and was with him during what ended up being his final night in the city.

"It's hard to get close to his heart as he protects himself so much and struggles to connect with strangers," she added, asking to use a pseudonym.

Hoang's behaviour that night did not strike Diana as all that unusual, she recalled.

The youngster walked out the door in the early hours of August 23. But within a day Diana was worried. She had not heard anything from him.

The mainland Chinese city of Shenzhen is seen from across the water in Hong Kong
The mainland Chinese city of Shenzhen is seen from across the water in Hong Kong AFP / May JAMES

Across the city, 11 other families were going through a similar ordeal.

Wong Wai-yin, a 29-year-old technician who was also on the boat, kept his escape plans secret from his family.

He is facing serious charges in Hong Kong of manufacturing explosives, and until he vanished had attended his bail appointments regularly.

After he disappeared, his wife and mother frantically searched his belongings, fearing the worst when they came across a message apologising should anything bad happen to him.

"I haven't washed his pillow since he disappeared," Wong's wife told AFP, requesting anonymity. "I am afraid that I will lose his smell some day."

Rumours soon began circulating among friends, relatives and activist networks that the group had been apprehended. But it was not until August 26 that confirmation came.

That day Chinese authorities announced that 12 Hong Kongers had been arrested making an "illegal border crossing".

Diana housed fugitive Hong Konger Hoang for three months before his capture by mainland Chinese authorities
Diana housed fugitive Hong Konger Hoang for three months before his capture by mainland Chinese authorities AFP / May JAMES

A pro-Beijing newspaper then published the names of those on board and details of the charges they faced in Hong Kong.

Details later emerged of an audacious but ultimately doomed attempt to reach Taiwan -- over 700 kilometres (430 miles) away, across the South China Sea.

As Beijing has cracked down on Hong Kong's democracy movement, democratic Taiwan has emerged as a sanctuary, quietly turning a blind eye to residents turning up without proper visas or paperwork.

Flying to Taiwan has become harder during the coronavirus pandemic and those charged with protest-linked crimes have often had passports confiscated, leaving a potentially perilous boat crossing as the only option.

The "Hong Kong 12", as the group were named, are the first known Hongkongers to be caught making the journey.

After trying to escape prosecution in Hong Kong, the fugitives now face a much starker legal battle inside authoritarian China's party-controlled judicial system, where conviction for those charged is almost guaranteed.

In Hong Kong, anyone arrested must be released if police fail to present them in court within 48 hours. The city's common law legal system, including trial by jury in an open court, is internationally respected.

Across the border the system is much more opaque.

Authorities only confirmed the group were in criminal detention in the neighbouring mainland Chinese city of Shenzhen 20 days after their arrests were announced.

Even now neither Beijing nor Hong Kong authorities have officially released their names.

In a joint press conference earlier this month, some family members said mainland lawyers they had appointed were repeatedly denied access to their clients, while others were warned not to talk to journalists.

The accused, the families were told, had been given "government-assigned" lawyers.

Hong Kong's pro-Beijing government has made clear it will only seek the return of city residents once they have been processed for any relevant crimes on the mainland.

Lu Siwei, one of the mainland lawyers working on the case, believes authorities will likely make a decision on whether to charge the fugitives in early October.

"If the prosecutors do not approve the arrest, the 12 Hong Kong residents should be granted bail or unconditional release by October 1," Lu wrote in his legal opinion.

"If the arrest is approved, they will continue to be detained and the possibility of conviction will be very high."