It's not often that the governments of major nations are so concerned about hunting down the authors of anonymous online letters.

But that is what's happening in China, as police have detained and questioned journalists and the families in China of overseas dissidents, in an apparent effort to find out who wrote a letter calling for President Xi Jinping to step down.

The incident is the latest chapter in a heated debate about the limits of free speech, under a leader who has tried to accumulate personal power and enforce ideological conformity far more than any of his recent predecessors.

The letter criticized precisely these actions. Claiming to speak for "loyal party members," it argued that Xi's personal accumulation of power had undermined the "collective leadership" of Communist Party elites and "weakened the power of all state organs."

It added that Xi's personal involvement in economic policymaking had caused volatility in financial markets, while his assertive foreign policies had antagonized China's neighbors. Other critics have leveled similar charges, but haven't gone so far as to call for Xi's resignation.

The letter urged Xi to resign for the sake of national stability, "and for your own personal safety and that of your family."

It was posted on overseas and Chinese websites on March 3, as legislators gathered in Beijing for the annual session of the National People's Congress.

Editors and managers at Watching.cn, a website backed by the Communist Party committee of the western region of Xinjiang and other investors, were the first to disappear after the website ran the letter.

Next, a freelance author named Jia Jia linked to one of the website's managers was detained at the Beijing airport as he tried to leave for Hong Kong. He was released on Friday.

On March 22, authorities in southern China also detained the parents and younger brother of Wen Yunchao, a New York-based blogger and activist.

Authorities' aim was apparently "to force me to cooperate with their investigation into this open letter," Wen said, speaking by phone from New York, "or even to force me to admit that I was involved with it."

"But I can't admit to something that has nothing to do with me," he added.

Wen said that his brother's wife went to local police to inquire about her husband's whereabouts, but was told that "they weren't handling this case, and they didn't know where this person is."

Wen said he believes that authorities have set up "a high-ranking task force" to handle the case.

Authorities also reportedly arrested relatives of Germany-based commentator and journalist Zhang Ping, who goes by the pen name Chang Ping, and told them to warn Zhang to stop criticizing the Communist Party and writing about the letter.

The hunt for the letter's author comes as Xi's first five-year term is drawing to a close. He is expected to get a second and final one at a Communist Party congress next year.

Xi has called for rank-and-file Communist Party members to avoid questioning central government policies, and for state-run media to unfailingly promote the party's views.

One brash real estate mogul who criticized Xi's demands last month had his social media accounts deleted, and quickly became the target of criticism by commentators in state media.

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