Beijing -- A kindergarten teacher has been arrested in eastern China after 23 students were sickened in a suspected act of deliberate poisoning, police said Tuesday. The teacher, identified by the surname Wang, is believed to have contaminated the children's food with sodium nitrite, the Jiaozuo city police said on their official microblog.

Overexposure to sodium nitrite, a white powder most commonly used in fertilizers, can be toxic and even fatal to humans. Police said they were still looking for a motive in last week's incident.

Schoolchildren in China have been the target of often fatal attacks by people bearing grudges or considered mentally ill.

Police officers are seen outside the gate of a kindergarten where a woman armed with a kitchen knife attacked children, in Chongqing, China October 26, 2018. REUTERS

In October last year, a knife-wielding assailant injured 14 children at a kindergarten in the western Chinese city of Chongqing. The attacker, a 39-year-old woman, was taken into custody but the motive for the assault was unclear.

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In April a 28-year-old former pupil allegedly seeking revenge for having been bullied stabbed nine pupils to death outside a middle school in northwestern China. Another 10 people were hospitalized with injuries resulting from the rampage in the rural area.

China tightly restricts private gun ownership, making knives and homemade explosives the most common weapons in violent crimes, but last week's incident in Jiaozuo wasn't the first suspected case of poisoning.

In 2002, 42 people, mostly schoolchildren, died after eating snacks laced with rat poison in the eastern city of Nanjing. The killer, who apparently was jealous of his rivals' thriving business, was swiftly sentenced to death and executed.