A postdoctoral urology researcher at UC San Francisco has been charged with attempted murder after admitting that he twice tried to poison a colleague by contaminating her drinking water with laboratory chemicals, a university spokeswoman said Tuesday.

Benchun Liu, 38, was arrested Nov. 7 after he allegedly told lab technician and research assistant Mei Cao that he had tried to kill her, police said.

“If he hadn’t have told her, we would have had no idea he was doing this,” UCSF Police Captain Paul Berlin said.

Liu and Cao work together in the Baskin Lab on UCSF’s Parnassus campus.

Liu, a Chinese citizen working in the country on a visa, is being detained in the San Francisco County Jail on an immigration hold. No bail information has been released.

According to police, Liu said he had been “stressed out” lately, but investigators have determined no other possible motivations for the crime.

Berlin said that in both poisoning attempts, Liu used a buffer agent that controls acid in lab solutions and turns water blue, but Cao drank it anyway.

“It was discolored; she noticed it,” he said. “She did swallow some of the contents. She claimed to have no reaction, but that is short term. We don’t know what the long-term impact will be on her body.”

Cao was examined at the UCSF Medical Center and released, Berlin said, adding that she was unable to say why Liu might have wanted to kill her.

“They had just a working relationship — no romantic relationship,” he said. “They both work in the lab, and I guess there is some competition in the lab. You could speculate for days. But what was exactly on his mind, only he knows.”