BEIJING, July 3 (Reuters) - Sloppy police work was partly to blame for a riot in a Chinese county in which about 30,000 people mobbed government offices and torched police cars, state media said on Thursday.

Residents in Weng'an county in Guizhou province went on the rampage demanding justice for the death of a girl police had ruled a suicide, not murder.

Police have denied that relatives of government officials had anything to do with the girl's death after allegations of a cover-up fuelled the unrest. Police insist the teenager killed herself by jumping into a river.

Saturday's violence was "fanned and exacerbated by local gangs and criminals, who were organized in sending gasoline, machetes, clubs and fireworks to aid the destruction", Xinhua news agency on Wednesday cited Luo Yi, the local police chief, as saying.

The official Guizhou Daily, citing the outcome of a meeting by senior provincial officials, said poor policing had created an atmosphere of lawlessness. This was one of the primary reasons the situation had spun out of control.

"Of the 600 to 800 criminal cases every year (in the county), only about half are solved," the newspaper said on Thursday.

"Some criminal cases cannot be solved in a timely way, there are many cases which have piled up, and the masses lack a sense of security," it added.

Some local officials and schools were also to blame for not paying enough attention to "moral education", the report said.

The police have already reopened the investigation into the girl's death.

China is counting down to the final days before the Beijing Olympics.

The government has started a nationwide stability push to prevent any domestic unrest upsetting the Games or spoiling China's quest to present itself as a harmonious nation. (Reporting by Ben Blanchard; Editing by David Fogarty)

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