Thallium is odourless and tasteless, and can be fatal in humans. The soft metal is used internationally in electronics manufacturing and for other purposes. It once was an ingredient in rat poison in the US, but has been banned for that use since the 1970s.
Yang's roommate, Juwan Royal, experienced progressively worse symptoms that included dizziness, shaking and vomiting.
A blood test later found Royal positive for thallium.
Royal, who has since graduated, continues to suffer symptoms, authorities said.
"This was over a period of time with thallium being added to foods and drinks in the refrigerator," Morganelli said. "He was getting worse all the time."
Royal called police after he drank a bottle of water in his room and his tongue began to burn, the Daily Mail reported.
He said he woke up Yang to tell him what happened and washed his mouth out, but his tongue remained sore for a few days.
Then on March 18 Royal got sick again and police were called.
Morganelli said Yang told officers that the milk in the refrigerator and Royal's mouthwash had changed colour. He told officers he believed someone was tampering with the belongings in their room.
Royal became sick again on the morning of March 29 when he began throwing up and shaking. He was taken to the hospital for treatment.
About a week later, graffiti that said 'n***** get out of here' was scrawled on Royal's bed and police were called.
Yang provided a written statement that said he left the room and locked the door. But Morganelli said police compared the graffiti with Yang's letters and saw similarities.
Yang was charged with ethnic intimidation, institutional vandalism and criminal mischief in that case.
It wasn't until the graffiti investigation that Royal took a blood test which tested positive for thallium, with 3.6micrograms per litre above the safe toxicity level for people, Morganelli said.
After Yang's computer was seized, he admitted to buying chemicals online but said it was intended to harm himself if his grades started to decline.
He did admit to mixing the chemicals with food and drinks in the dorm refrigerator.
"Initially, Mr. Royal was dumbfounded by this as everyone else, because he believed they had a fairly cordial relationship as roommates," Assistant District Attorney Abe Kassis said.
The students were roommates for several years. Yang is no longer enrolled at Lehigh, and his student visa has been revoked.
