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Qian Jun, head of the disease control and biological security office with China's Academy of Military Medical Sciences, said the academy's Beijing lab found no needle injury samples were tainted with radioactive substances, toxic chemicals or HIV virus.
The samples were not contaminated with other dangerous viruses or substances either, such as anthrax bacillus, yersinia pestis, francisella tularensis, brucella and botulinum toxin, Qian told a press briefing.
Local and military medical experts have rechecked about 250 victims and found no clearly worsening wounds or serious illnesses, he noted.
By Sept. 4, local authorities had confirmed 531 victims of hypodermic syringe stabbings in Urumqi, 171 of whom showed obvious syringe marks. The majority of the victims were of the Han ethnic group.
Tens of thousands of angry and panic residents in Urumqi took to the streets last week, protesting against needle attacks and demanding security guarantees.
Qian suggested offering more psychological counselling to ease anxiety and depression of the victims as many are haunted with lingering fears of hidden infections.
The Urumqi General Hospital affiliated to the Lanzhou Military Area Command has arranged three psychological experts and opened four counselling hotlines to help ease victims' fears and panic.
Wang Wenxian, deputy director of the Urumqi municipal public security bureau, said the needle stabbings did not cause serious damages to the victims' health, but they caused public panic and disturbed social order.
The acts violated China's Criminal Law and should be harshly punished accordingly, Wang told reporters.
A court in Urumqi said three Uygurs were given jail terms ranging from seven to 15 years Saturday over syringe stabbings or threatening to use needle attacks for robbery.
Wang added that more police and armed police forces would patrol on the city's streets and those who offer tip-offs for needle attackers would receive rewards.
He also urged the attackers to surrender to the police, saying those who surrender or report others' crimes could receive lighter punishment.