An expert urged open investigations of a string of violent incidents recently, after three more people were stabbed to death and five injured by a mental health patient in Shenzhen on Monday.

The 40-year-old man with a history of mental health problems allegedly attacked the people on a street in Luohu district at around 10 am on Monday, local police said in a statement on its official weibo micro blog page.

Police cordon off an area where a man, who allegedly suffers from mental illness, stabbed three passersby to death and injured another three in Luohu district, Shenzhen, on Monday. Zhang Guofang / for China Daily

The man, who has been visiting hospitals regularly and taking medicine for his condition since 1991, had a psychotic episode on Monday and his father could not stop him from leaving their home, police said.

The man allegedly took a knife from a snack bar near his home and started to assault passersby. Police said he also harmed himself and had been taken to a hospital.

The owner of the snack bar suffered minor injuries when she tried to stop the man.

She told China Daily that she is still in shock, as two people were killed in front of her shop, but she did not want to talk about the attack.

Zhang, the male owner of a store near the crime scene, told China Daily the man entered the snack bar kitchen saying he needed to wash his hands, but took a knife and cut the snack bar owner before running out to attack more people.

A woman surnamed Ye, who works at a jewelry store, and a security guard surnamed Zhang, who works at Shenzhen Middle School, both said the man hurt himself as police were trying to control him.

The identities of the victims have not yet been released.

A string of violent attacks has been reported in recent months. Beijing alone had two violent attacks in July, in which suspects reported to be suffering from mental illnesses caused three deaths and injured three other people.

A man stabbed two pedestrians to death in Chaoyang district on July 17. Five days later, another man attacked four people with a knife at a Carrefour supermarket in downtown Beijing, killing one and injuring three.

Li Meijin, a professor of criminal psychology at the People's Public Security University of China, said the reasons for such violent incidents are unclear as not much information is made public.

"Usually it's in spring that psychotic episodes happen easily. I don't know why they also happen often during the summer. Some may happen accidentally, and others may reflect some social problems," Li said.

"If the suspects do have mental disorders, an investigation is needed to provide proof and make them public. We also have to think about ways to prevent people with mental disorders and violent tendencies from harming others," Li said.

"An investigation is also needed to know if those suspects were treated unjustly before. Punishment on them is not meaningful, if the reasons behind their violent behavior are unknown."

Li suggested special committees should be established to investigate serious violent crimes or a series of similar crimes, and information on those cases should be made available as a study reference.

Wang Yun and Zhu Jing in Shenzhen contributed to this story.