The suspects include two 37-year-old men believed to be the assailants who were arrested on the Chinese mainland Sunday and seven accomplices, ages 30-57, picked up in Hong Kong Wednesday, the South China Morning Post reported.
"At the moment, we believe that the assailants were hired and they have triad [organized crime] background," Hong Kong Police Commissioner Andy Tsang Wai-hung told reporters.
Authorities allege the suspects had various roles before and after the Feb. 26 attack on former Ming Pao chief editor Kevin Lau Chun-to.
"According to information in hand, there is nothing directly linking the attack to [Lau's] journalistic work," Tsang said, adding the investigation was ongoing and further arrests were possible.
Lau was attacked by a passenger who leaped from a motorcycle and slashed him repeatedly from behind as he was getting out of his car to go for breakfast in Sai Wan Ho.
Lau's removal as editor in chief at the 55-year-old Chinese-language daily in January had caused international concern with thousands of people taking part in a march calling for freedom of the press.
Lau was replaced by a Malaysian journalist.