Chongqing police yesterday doubled their reward to 600,000 yuan (HK$680,000) in efforts to find an unidentified attacker who shot dead a PLA sentry and stole his sub-machine gun last month. About 10,000 policemen working on the case for two weeks have failed to make progress. The latest bounty offer came together with the release of four blurry pictures of the attacker that were captured by closed-circuit television at the scene and limited information about the attacker, China News Service reported. Police said the attacker might have a criminal record, or have served in the armed forces or at least received some military training. '[We] initially found that the attacker is a 1.7-metre-tall man aged between 18 and 45, medium build, wore a light-coloured shirt, dark trousers, and a silk hat. He hid the stolen sub-machine gun in a white crocheted bag,' the Gaoxin district police bureau said on a wanted announcement posted all over the municipality's streets yesterday. Chongqing authorities said any tip-off enabling the police to solve the case would be entitled to a 300,000 yuan reward from the Chongqing Public Security Bureau and another 300,000 yuan from the bureau in Gaoxin district, where the rare assault happened. It said witnesses who could give the suspect's address or related information would also be rewarded. The announcement gave four contact numbers and an e-mail address for the special investigation team. On March 19, a masked gunman shot dead the People's Liberation Army sentry outside the garrison and fled with the soldier's assault rifle. Authorities have classified it as an act of terrorism. Chongqing police have failed to make any progress in the past 15 days, although they announced a six-month crackdown on fugitives after the assault. Police have not received any valuable reports, even though a 300,000 yuan reward was offered the day after the assault. The municipality also announced the results of a random survey of about 20,000 households yesterday, saying 90 per cent of its residents had voted Chongqing a safe place, the Chongqing Evening News said. The survey, conducted by the Chongqing public security administration office, said fewer than 10 per cent of the interviewees believed the city was less safe now. Chongqing party boss Bo Xilai had played down the shooting incident, urging the public to stay calm. He described the case as an 'isolated emergency incident'. He said such incidents were inevitable in big cities. 'We should step up precautionary measures and educate the public,' he said. The assault came as the mainland faces a year of sensitive anniversaries, including the 50th anniversary of a failed Tibetan uprising and the 60th anniversary of Communist rule in Uygur-populated Xinjiang.