HONG KONG (AP) — Hong Kong police on Thursday said they arrested 11 people on suspicion of assisting offenders who are believed to be the 12 Hong Kong activists detained at sea by mainland Chinese authorities while attempting to flee the city last year.

District councilor and lawyer Daniel Wong Kwok-tung posted on his Facebook page early Thursday that national security officers had arrived at his home. He was later taken to his office, where police conducted a search.

Wong, a member of the Democratic Party, is known for providing legal assistance to hundreds of activists arrested during antigovernment protests in 2019.

The fresh wave of arrests comes a week after 55 activists were apprehended in the largest move against Hong Kong’s democracy movement since Beijing imposed a new national security law last June to quell dissent in the semi-autonomous Chinese territory.

The clampdown has intensified concerns that Beijing is asserting more control over the city and breaking its promise of Hong Kong maintaining separate civil rights and political systems for 50 years from the handover from Britain control in 1997.

Police said in an emailed statement that those arrested Thursday were eight men and three women aged 18 to 72. The identities of those arrested were not revealed.

Local media reports said those arrested are suspected of helping the 12 Hong Kong activists who were detained at sea by mainland Chinese authorities while attempting to sail to Taiwan last August. Some of the fugitives were wanted in Hong Kong for offenses related to the 2019 protests.

Last month, 10 of the activists detained at sea were sentenced to prison in Shenzhen for illegally crossing the border, with sentences ranging between seven months to three years. The two other detainees, who are minors, were handed over to Hong Kong and could face further charges in the city for absconding, authorities said.

Hong Kong has jailed several pro-democracy activists in recent months, including Joshua Wong and Agnes Chow, for their involvement in the anti-government protests. Others have been charged under the national security law, including media tycoon and outspoken pro-democracy advocate Jimmy Lai.

The security law criminalizes acts of subversion, secession, terrorism and collusion with foreign powers to intervene in the city’s affairs. Serious offenders could face up to life imprisonment.

Last week, the foreign ministers of Australia, the United States, Great Britain and Canada issued a joint statement expressing “serious concern” about the arrests. They said “it is clear that the National Security Law is being used to eliminate dissent and opposing political views.”