NANJING - Police in an east China city Thursday doubled the reward for providing clues about the man behind a high-profile armed robbery who is also suspected to have committed a series of murders and robberies in other cities over the past seven years.

Police in Nanjing, capital of Jiangsu province, are offering 200,000 yuan ($31,700) for information that leads to the arrest of the person who gunned down a man in front of a bank in a busy street last Friday and fled with 200,000 yuan that the victim had just withdrawn from the bank, the city's police said.

Thousands of police have been mobilized to conduct a "blanket" manhunt in Jiangsu, they added.

The suspect, reportedly a former armed police officer in his 40s, is believed to have used a gun to kill six people and injure another two in six robberies that took place in Changsha, the capital of Hunan province, and Chongqing municipality since 2004.

Civilians in China are banned from possessing firearms and the case has shocked the public as the suspected serial killer appears to have easily dodged police hunts and repeatedly broken the law over the past seven years.

More theft and robbery cases are usually reported before the Chinese New Year, as people are making more bank transactions and migrants to the cities are carting their money back home for Spring Festival.

The festival, or Chinese New Year, the first day on lunar calendar, falls on January 23 this year.

Police in Changsha also have raised security levels and conducted checks on hotels, Internet bars, entertainment venues, and rented houses throughout the city.