A former chemistry major at Lehigh University, who allegedly vandalized his longtime roommate's property with racist graffiti, was charged with attempted homicide Thursday after he also allegedly poisoned the roommate over several months by mixing toxic amounts of thallium into his food and drink, prosecutors said.

North Hampton County District Attorney John Morganelli announced the charges at a press conference, calling the behavior "weird and bizarre."

Yukai Yang, 22, who turned himself in Thursday, is no longer enrolled at Lehigh, a university spokeswoman confirmed to USA TODAY. Yang is also a Chinese national and had his student visa revoked, prosecutors said.

At the press conference, live streamed by WFMZ-TV, Morganelli said Yang poisoned roommate Juwan Royal using small amounts of the metal over a several months, adding it to his milk, water and mouthwash.

In February, Royal felt burning sensation on his tongue after drinking bottled water, he told police. He woke up Yang then washed his mouth out in the bathroom, where Yang allegedly said, "So this substance that they are putting in your drink is colorless, odorless and dissolves in water," according to prosecutors.

Police responded multiple times in March when Royal experienced symptoms, including dizziness and vomiting, and he was taken to the hospital once.

In early April, police responded to the room again for vandalism and damage to Royal's property that included a racial epithet, prosecutors said. Royal is African American, Morganelli said.

Yang at the time was charged with crimes related to the damaged property — including ethnic intimidation — after police compared his handwriting to the vandalism. Morganelli said that case is still being prosecuted.

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Police later did chemical testing on Royal's blood, which tested positive for thallium at a rate of 3.6 micrograms per liter, above the safe limit for humans.

Had the levels been higher, there was a "strong chance" Royal could have died, and thallium, a "very effective poison," has been used to kill in other cases, Assistant District Attorney Abe Kassis said.

Yang later admitted to purchasing chemicals online, including thallium, to harm himself, and said he did mix them into Royal's food and drink, Morganelli said. Morganelli couldn't say yet whether the metal was purchased legally.

Both students were seniors during the period when the incidents took place, and had roomed together in previous years.

"Initially, Mr. Royal was somewhat as dumbfounded by this as anyone else," Kassis said. "He believed they had a fairly cordial relationship as roommates." He is still feeling symptoms from the poisoning, prosecutors said.

In a statement, Lehigh University said it was working with authorities in the case.

"From the outset, our concern has been the health and safety of the victim of these alleged behaviors and, as such, Lehigh staff and faculty have been providing support, services and assistance," the statement read.

Follow Ryan Miller on Twitter @RyanW_Miller.