A Chinese woman who was the victim of an unsolved poisoning case in 1994 has died, aged 50.

Zhu Ling was a chemistry student at Beijing’s Tsinghua University when she was poisoned with the highly toxic chemical thallium.

It left her paralysed, almost blind and with brain damage, and she required 24-hour care from her parents.

No-one has been charged in the case. Her classmate and roommate, Sun Wei, was investigated but not charged.

Ms Sun was investigated by police in 1997 but was cleared as a suspect due to a lack of evidence.

She has repeatedly taken to social media to declare her innocence, and also changed her name.

In late 1994, Zhu Ling began experiencing stomach pains and hair loss and she fell into a coma months later.

Doctors later diagnosed her with thallium poisoning, a soft metal which dissolves in water and is odourless and tasteless.

Reports have claimed Ms Sun had access to the thallium compound but she has said she was not the only student with access to the toxic substance.

Zhu Ling’s family and supporters have suggested a possible motive was that Ms Sun was envious of her beauty and her musical and academic achievements.

Ms Sun has denied personal animosity between herself and Ms Zhu.

A 2013 petition demanded the US investigate Ms Sun and have her deported from the United States, where she was living.

The petition described Ms Sun’s family as having powerful political connections, and it was alleged she “had the motive, and access to the deadly chemical”.

The White House responded to the petition and said it declined to comment on the request, but said Zhu Ling’s poisoning “was a tragedy”.

It read: “No young person deserves to suffer as she has, and we can understand the heartbreak of those close to her.”

In 2013, the Beijing Public Security Bureau defended its investigation but said the amount of time that had passed, alongside the small amount of evidence, limited its ability to reopen the case.

Media reports have said there was speculation that Ms Sun was protected from prosecution because of her grandfather Sun Yueqi, a senior Chinese official, and another relative, a former vice mayor of Beijing.

Ms Sun said her grandfather had died by the time she was questioned by police.

The police rejected claims that its inquiry had been influenced by others. In 2013 police said: “The dedicated investigation team worked according to law, and the investigation was never compromised or interfered with in any way.