Hong Kong police are investigating the parties – including possible ringleaders – involved in arranging the escape of 12 wanted Hongkongers who were subsequently caught at sea while fleeing to Taiwan, as the force revealed for the first time details of their getaway.

The force also said on Saturday it was told by Guangdong authorities on Friday that the Yantian branch of the Shenzhen Public Security Bureau would submit the case for arrest approval, which meant the 12 could continue to be detained on the mainland.

“We are also investigating which local smuggling syndicate provided a speedboat for the 12 suspects and arranged the smuggling route to Taiwan,” a police source said.

He noted it was unlikely that the speedboat used in the escape would have carried the group directly to Taiwan, saying a bigger vessel was probably waiting somewhere outside Chinese waters, waiting to pick them up and complete the journey.

“Of course, it is possible some of the 12 suspects might also have helped organise the smuggling activity,” he said. “We will also look into this.”

The source said police were investigating all possible clues in connection with this case.

He said officers from the force’s liaison Bureau would also maintain close contact with their mainland counterpart.

The detention of the 11 men and one woman, aged 16 to 33, in Shenzhen’s Yantian’s district for a month sparked a diplomatic row between China and the United States after the group was captured at sea on August 23 while sailing to Taiwan to seek political asylum.

Most of the 12 were linked to the city’s anti-government protests last year. One of them, activist Andy Li, 29, was arrested under the city’s sweeping new national security law.

In a statement issued on Saturday night, Hong Kong police disclosed for the first time some key findings of initial investigations.

“Investigation revealed that the 12 persons boarded a speedboat arranged by a smuggling syndicate in the Po Toi O pier in Sai Kung, New Territories, Hong Kong at around 7am on August 23,” the statement read.

“The speedboat was driven by one of the suspects. They planned to flee to Taiwan via mainland marine waters in order to evade criminal responsibility in Hong Kong. They have paid the syndicate a certain fee before boarding the speedboat in Hong Kong for the smuggling arrangements.”

Police said they received a reply from the Guangdong Provincial Public Security Department on Friday, stating that the coastguard had intercepted a boat suspected of crossing the boundary illegally on August 23.

“It was suspected that the speedboat … entered mainland waters through [the] southeastern side of the boundary of Hong Kong waters at around 7.30am on the same day. The speedboat in question later passed through the effective detection range of the systems,” the force said.

“The announcement issued by Guangdong Coast Guard stated that at around 9am on August 23, a speedboat suspected of crossing the boundary illegally was intercepted in marine waters under its jurisdiction.”

The mainland authorities told police the 12 were arrested for the offence of crossing the boundary illegally. After initial investigation, the Yantian branch of the Shenzhen public security bureau detained the 12 suspects for further investigation.

Hong Kong police said they were “investigating in the direction of whether someone organised or committed cross-boundary crimes” in the case.

Ronny Tong Ka-wah, an adviser on the Executive Council, the city leader’s de facto cabinet, said the smuggling syndicate could be investigated for organising or committing cross-boundary crimes.

“If someone among the 12 advocated others in joining the escape, he or she could also be probed for organising cross-boundary crime,” he said.

Tong said under the city’s laws, there was room for the 12 to be sent back to Hong Kong. “If mainland authorities approve their arrest and convict them, they can only come back after they served their sentence,” he said.

Democratic Party lawmaker James To Kun-sun said he was not interested in what Hong Kong police officers were investigating.

“I won’t let that distract me from the fact that the 12 have no freedom to choose their lawyers in Shenzhen,” To said. “Hong Kong authorities should just disclose all relevant marine radar data and other images to back up what they said. Even murder suspects could be returned from the mainland to Hong Kong in the past, why can’t they do the same this time?”

Council Front lawmaker Eddie Chu Hoi-dick also said Hong Kong police should not be “a mouthpiece of the Communist Party”.

He added: “Under mainland laws, arrest approvals must be applied for within 30 days of suspects’ arrest. But the 12 Hong Kong people have disappeared for 35 days already.”

In a statement, the family members of the detainees also questioned whether mainland authorities were holding the 12 unlawfully.

Lu Siwei, a mainland human rights lawyer representing a female detainee, said he believed the case was sent to Shenzhen authorities for arrest approval after the 30-day limit expired on Thursday.