The man at the center of the country's third-ever thallium poisoning case has confessed to police that he poisoned three classmates in an act of retaliation.

Three college freshmen were poisoned on May 31 while having supper with the man, surnamed Chang, in their school dining hall at Xuhai College, part of China University of Mining & Technology, in Xuzhou, Jiangsu Province.

Chang was arrested on June 12, after police found traces of thallium on the bowls of the victims but not on his.

The China Center for Disease Control and Prevention (CDC) confirmed the three were suffering from thallium poisoning on June 10, after collaborative efforts by Beijing, Hebei and Jiangsu CDCs.

The three students are still being treated at Beijing's Chaoyang Hospital but are said to be in a stable condition.

An unnamed source said Chang was an introvert who often felt his classmates treated him unfairly.

Chang himself told police his hatred for his peers escalated after being constantly alienated and ignored by them during leisure breaks.

He said that on the night of May 31, he invited the three to the school dining hall and clandestinely poured thallium liquid into their bowls.

The police are still investigating how Chang was able to get hold of the highly toxic heavy liquid, whose purchase involves detailed and complicated authorization procedures.

Death by thallium poisoning is known to be slow and painful.

The country's first recorded case of thallium poisoning also happened on a college campus and remains unsolved to this day.

The victim, Zhu Ling, who was studying chemistry at Tsinghua University, first fell ill in December 1994. Her condition soon became serious, but doctors could not formulate a diagnosis.

By the time they discovered Zhu had been poisoned with thallium, her central nervous system had already been badly damaged.

Today, the 34-year-old is completely paralyzed and has lost almost all of her eyesight and mental capacity.