Hong Kong's chief executive says he will take up the alleged kidnapping of five booksellers with the central Chinese government in Beijing.
CY Leung added he would review the way Hong Kong authorities are informed if a citizen breaks Chinese law.
One of the five, Lam Wing-kee, now back in Hong Kong, says he considered ending his life while he was detained.
Many believe the booksellers were detained because they sold gossipy books about China's leaders.
One of the men, Gui Minhai, is still in custody.
China and the booksellers
1. Lui Bo, general manager. Went missing: Shenzhen, 15 October 2015 Returned: March 2016
2. Cheung Jiping, business manager. Went missing: Dongguan, 15 October Returned: March 2016
3. Gui Minhai, co-owner. Went missing: Thailand, 17 October Still missing
4. Lam Wing-kee, manager. Went missing: Shenzhen, 23 October Returned: June 2016
5. Lee Po, shareholder. Went missing: 30 December - he says from the mainland, Mr Lam says it was from Hong Kong Returned: March 2016
The chief executive said he believed there was "room for review and possibly improvement" in the system of notifications designed to let the Hong Kong government know if a person from the city breaks the law in China.
His comments reflect the fact that the Hong Kong authorities were not initially informed of the booksellers' arrest and detention.
Mr Lam was arrested while visiting the Chinese city of Shenzhen in October last year. He says he was taken, blindfolded, to the eastern city of Ningbo, where he was held until March.
In March, when three of his colleagues were released and returned to Hong Kong, Mr Lam was transferred to a room in the city of Shaoguan in Guangdong province, where he said he enjoyed more freedom.
He returned to Hong Kong last week, and he claims the Chinese authorities were expecting him to return to China with details of his customers, something he does not now plan to do.