Beijing police said Wednesday that they had failed to solve a poisoning case in which Tsinghua University student Zhu Lingling, formerly known as Zhu Ling, was paralyzed 19 years ago, due to the lack of evidence needed to convict the suspect. This caused netizens to call on the police to make public the investigation results.

This is also the first time that local police have ever responded to a public request for reinvestigating the case, in which Zhu's roommate, Sun Wei, was alleged to have orchestrated the poisoning.

According to a post on the capital police's Sina Weibo account on Wednesday, the police said they had visited and interrogated over 130 people related to the case, as well as carried out inspections in over 100 research institutes across the city that possessed thallium.

However, we failed to find direct evidence to convict the suspect, as the case was reported to the police half a year after Zhu was first poisoned, and by then the evidence had all been destroyed, said the statement.

"This is almost the same as what they told us back in 1998," Wu Chengzhi, Zhu's father, told the Global Times Wednesday. He added that his greatest concern was to take good care of his daughter but that they had never received any compensation from the local government or the university.

Zhu was first hospitalized due to severe stomach pain and hair loss in December 1994. She was then rushed to hospital on April 28, 1995 for a second time, when she was diagnosed with thallium poisoning. The case was only reported to the police by the school in May.

There had previously been speculation that the roommate, Sun, who has a prestigious family background, was responsible but charges were never laid and she was set free after an interrogation in 1995.

"Local authorities should release the details on their investigation to convince the people," a Beijing media worker who has been following the case for months, who asked not to be identified, told the Global Times.

"We should respect the justice of the law. It is not abnormal that the police failed to solve the cases due to some objective factors, for example, the limits on the investigative technique and the lack of solid evidence," said Qu Xinjiu, an expert on criminal law from the China University of Political Science and Law.

Zhu's case regained huge attention among the public after Huang Yang, a 28-year-old graduate student at Fudan University, was poisoned to death by his roommate in late April.

More than 100,000 netizens have signed in an online petition on the website of the White House, demanding the US government investigate the case and deport Sun, who is currently in the US.