SAN FRANCISCO -- Prosecutors Wednesday charged a University of California at San Francisco lab researcher with attempted murder, poisoning and assault with a deadly weapon after he allegedly tried to poison a female coworker because he was "stressed out."

Ben Chun Liu, 38, a Chinese post-doctoral student in the United States on a work visa, was arrested Friday, according to the university.

He remained in custody today on a no-bail immigration hold, according to the Sheriff's Department.

Liu is scheduled to be arraigned on the charges Thursday morning in San Francisco Superior Court, according to the District Attorney's Office.

Liu allegedly poured a lab chemical into the drinking water of 44-year-old lab technician Mei Cao on two occasions in recent weeks, university police Capt. Paul Berlin said. The first incident occurred Oct. 23, he said.

Both worked at the university's urology department, according to the school.

Berlin said Cao noticed there was something in the water after drinking some of it. After Liu allegedly admitted to her that he had tried to poison her, she notified police, Berlin said.

Cao was examined at the UCSF Medical Center and later released, and is in good health now, Berlin said.

Police are working with the lab to try to determine the chemicals that were allegedly put in the water, and any possible long-term effects, Berlin said.

"They know it's not good for you," said Berlin. "It's not like taking vitamins."

A motive for the alleged poisoning is still not known.

Berlin said the two had both worked together at the lab for at least a year, did not have a romantic relationship, and that Liu had never expressed any hostility toward Cao.

Liu would only tell police that he had been "stressed out," Berlin said.