Police in Guizhou province last month detained 217 people, including 39 gang members, alleged to have been involved in a violent protest following a teenage girl's death in Weng'an county.
Altogether, 355 suspects, 90 of whom are "gangsters", were found to be involved in the riot, but police are still seeking other suspects in hiding, Peng Dequan, vice-director of the Guizhou provincial public security department, said.
More than 500 people had been investigated as of Saturday, he said.
The detentions were made after police said on July 2 they had launched a four-month crackdown on gangs accused of inciting violence over schoolgirl Li Shufen's death.
A third and final autopsy on Li, 17, concluded last week showed that she drowned.
On June 28, the same local police finding angered her family and relatives, who had contended she was raped and killed, and finally spurred a gathering of some 30,000 people and a subsequent violent protest at the Weng'an county government and public security bureau.
Government buildings were set on fire by people who senior provincial authorities later described as "fanned and exacerbated by local gangs and criminals".
More than 150 police and protesters were injured, but no deaths were reported. About 160 offices and more than 40 vehicles were torched.
In a related development, officials from politics and law committees at all levels in the province will be open for petitions from today until Aug 31 to address similar problems at the most grassroots level.
Leaders at every level of the office must "directly listen to claims of the masses" by taking their petitions, or through field research and appointments, a document released by the provincial politics and law committee last Friday said.
"Leading officials must take on the cases themselves, conduct thorough research to find solutions, and take part in the investigations, negotiations and settlement, until the case is closed and the matter settled," it said.
Four Weng'an officials, including the county's chiefs of the Party, government, and police authorities as well as the politics and law committee, were sacked after the riot for "severe malfeasance".
Guizhou Party chief Shi Zongyuan said the deep-rooted reasons behind the protest were local authorities' use of "rude and roughshod solutions" to address a number of issues.
"Some officials neglected their duties but resorted to the police force to resolve dispute, which led to strained relations among officials and the people, and police and the public," Shi said.
(China Daily July 15, 2008)