Beijing police responded to the controversy surrounding an unsolved poisoning case nearly two decades old for the first time on Wednesday.

The case, which happened at an elite Beijing university, has recently gained much attention from Chinese netizens.

Beijing police announced on Sina Weibo that the investigations into the poisoning case of the former Tsinghua University student Zhu Ling have been closed for lack of evidence.

"We appreciate the public's concern for the 'Zhu Lingling Case' (Zhu's official name), and we're sorry that the case failed to reach a conclusion," the police said following a heated online campaign that called for a new investigation.

Zhu, a 21-year-old student from Tsinghua University, was poisoned with thallium, a heavy metal that was introduced in her drinking water in 1994. Her life was ultimately saved, but she suffered serious neurological damage and permanent physical impairment.

Although no evidence has ever been presented publicly and no charges brought, online speculation suggested that Sun Wei - one of Zhu's roommates - was behind the incident. There have also been claims that Sun was not charged with the crime because a number of her relatives were powerful officials.

Sun has previously denied the accusations. The police statement highlights the fact that the suspects' families didn't interfere in the investigation.

According to the statement, the investigation of the case started on May 5, 1995, immediately after they received the report from Tsinghua University.

"We formed a special team to investigate the case and investigated 130 people that were related to Zhu's daily life, while also carrying out investigations on more than 100 companies and units that have thallium business," the statement said.

However, Zhu's lawyer Zhang Jie has a different view.

"The police's response is just trying to divert attention to the fact that they couldn't solve the case," Zhang said.

"But still I consider that it's a kind of progress to get a response from the police."

"Anyway, even though there are difficulties ahead, Zhu's parents and I will never give up trying to bring the criminals to justice," he added.

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