HONG KONG, July 26 (Reuters) - One of the Hong Kong democracy activists who was returned from a Chinese jail after being captured at sea last year was ordered detention in a training centre for under-20-year-olds on Monday for attempted arson and possessing dangerous objects. Hoang Lam Phuc, 17, was among a group of 12 people intercepted by mainland authorities in August 2020 on a boat believed to be en route to Taiwan, a case which drew international attention and concern over their treatment in China. He has been in custody either in China or in Hong Kong since then. Training centres are an alternative to imprisonment for people aged 14 to 20, in which authorities focus on rehabilitation and offenders are trained in a trade. The minimum period of detention is six months and the maximum is three years, but the duration is ultimately decided by the Commissioner of Correctional Services, who will consider the conduct of the offender during detention. Hoang had pleaded guilty on the charges of attempted arson and possessing anything with intent to destroy or damage property, which were related to Hong Kong"s mass pro-democracy protests in 2019. Judge Frankie Yiu in the District Court said Hoang "completely ignored the law and was irresponsible," but he took into account that he was only 16 when he committed the crimes and that he was honest in his confession. Earlier on Monday, Hoang also pleaded guilty in a separate case for "intending to pervert the course of public justice," by fleeing the Hong Kong jurisdiction, a crime which will also be covered by the training centre detention. At the time of their capture, all 12 were facing charges in Hong Kong over the 2019 protests. In China, they were sentenced to between seven months and three years for illegally crossing the border, or organising the crossing. Hoang was one of two minors sent back to Hong Kong in December and has been detained in the territory since then. Eight others were released from a Chinese prison in March and are also in custody in Hong Kong. Two activists with longer sentences are still in a Chinese prison. During the detention of the 12 in the southern Chinese city of Shenzhen, mainland authorities denied their families and lawyers access, insisting they be represented by officially appointed lawyers, provoking criticism from rights groups. Diplomats and journalists were not allowed to attend their trial in China. Chinese authorities had said their case was handled "in accordance with the law."