Chinese police are tracking suspects that are believed to be responsible for beating to death a reporter who was investigating a coal mine in north China's Shanxi Province, local authorities reported yesterday.

The Shanxi Provincial Security Bureau has dispatched more than 70 officers to investigate the case and they have made some breakthroughs, officials said, without elaborating.

"Lan Chengzhang, who worked for the Beijing-based China Trade News, and a taxi driver he had hired, were attacked on Jan. 10 at a coal mine in Hunyuan county of Datong city," a local police official said.

"The coal mine owner sent more than 20 thugs in their twenties, from Datong, to beat up the two men," he said.

Lan died of a brain haemorrhage later in the hospital and his colleague is undergoing treatment for two broken legs.

The incident has provoked online tirades from Chinese netizens at the treatment of the reporter and accusations of blackmail from local officials have also poured fuel on the fire by accusing Lan of posing as a journalist in order to blackmail the coal mine, which had no production license.

"It is inappropriate to discuss whether or not Lan was a registered journalist now. The most important thing is to arrest the assailants and deal with them according to the law," said Da Li, a reporter with a local TV station.

Datong city government spokesman Gu Shengming said Lan had no press card and his interview was not officially approved. However, he admitted that Lan had been hired by the newspaper.

"Lan has been temporarily hired by the newspaper's branch in Shanxi for a short period. It is still not clear when he went to the coal mine or how the accident happened," Gu said.

"A vice president of the newspaper has arrived in Datong to investigate the case," he added, without giving the official's name.