Li Baiguang, a Christian human rights lawyer who had met regularly with high-level officials in the United States, died at the Number 81 People's Liberation Army Hospital in Nanjing, eastern China, a source with direct knowledge of the case told Reuters.
"After a sudden outbreak of illness, the efforts to save him failed, and he passed away. He had not done a health check recently, so we do not know if there was a long-term cause," the source said, requesting anonymity due to the sensitivity of the case.
The hospital where Li died declined to comment, saying it could not divulge personal details of patients.
In 2008, Li was given an award by the National Endowment for Democracy, a Washington-based nonprofit organisation funded largely by the U.S. Congress, for his work defending arrested Christian pastors. Li is survived by his wife and eight-year-old son.
Bob Fu, a U.S.-based Chinese-American who was a close friend, said the death appeared suspicious, given a number of high-profile rights activists have died from liver failure while in custody in recent years.
"Li did not drink alcohol and he did not smoke. For someone's liver to fail overnight at the age of 49 is highly unbelievable," he said.
China's public security ministry did not immediately respond to a request for comment on Li's case or allegations of neglect.
China has vowed to tackle problems of forced confessions, physical abuse and torture in its criminal justice system. It also regularly rejects criticisms of its human rights record, saying that China is a country with rule of law.
Li had been detained by the authorities before, including in October last year, but it was unclear if he had been questioned by authorities more recently, Fu added.
In 2017, ill treatment by the Chinese authorities may have directly contributed to the deaths of Nobel Peace Prize winner Liu Xiaobo and rights activist Yang Tongyan, according to a report released on Monday by the Hong Kong-based advocacy group Chinese Human Rights Defenders.
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