Chinese authorities have detained a dozen activists from Hong Kong who were attempting to leave the territory via speedboat, according to people familiar with the attempt and the individuals captured, as Beijing intensifies a campaign to seek out protest leaders and others resisting the Communist Party’s tightening grip.

At least one of the people on board the boat, seized Sunday by the Chinese Coast Guard, was an activist who was being investigated under the city’s new national security law, said one of the people familiar with the capture.

The group was apparently trying to flee to Taiwan, said a second person familiar with the episode. More than 200 Hong Kong protesters and activists have sought refuge in Taiwan over the past year. The detentions Sunday were the first confirmed case of such activists being caught by Chinese authorities at sea.