(Corrects description of Sun Wei’s grandfather in third paragraph.)

Why did China’s leading social-media platform recently ban users from performing searches for a woman poisoned in 1995? Attempts to answer that question -- and to censor the answers -- have sparked some of the most politically potent online commentary on Chinese leadership, privilege and corruption in recent memory.

The details of the almost two-decade-old case are sordid and murky. In 1995, Zhu Ling was a promising undergraduate at Beijing’s elite Tsinghua University when she came down with a mysterious illness that was thought to be poisoning via thallium, a toxic element once used as rat poison. This finding soon led to a suspect: Sun Wei, a roommate of Zhu’s who happened to be one of the few undergraduates at Tsinghua to have access to thallium in a laboratory.

Most important for the politically minded Chinese netizen, Sun Wei was the granddaughter of a high-ranking official who was thought to be close to then-President Jiang Zemin. In 1997, Sun was detained by police for questioning for eight hours but not arrested. Soon after, the case was closed, and Sun reportedly fled to the U.S., where it’s rumored she’s married with kids (enterprising microbloggers have tried to keep tabs).

Meanwhile, Zhu, permanently disabled, lives with two elderly parents ill-suited to care for someone that Hong Kong’s South China Morning Post described as “a 200-pound, paralyzed, diabetic, almost-blind woman with the mental capacity of a six-year-old.”

Over the last month, the tale has re-emerged as a populist cause celebre. The trigger was the early April fatal poisoning of a student at Shanghai’s Fudan University by another student, which evoked memories of the 1995 incident. Over the course of April, Zhu’s name became an increasingly popular topic of conversation and a proxy for anger at official privilege. Few offenses inspire Chinese online ire like the use of privilege -- especially by children of those in power -- to avoid the consequences of criminal behavior.

It’s not clear that anyone intervened in Zhu’s case, but that hardly matters in a China accustomed to rumor. Zhu’s angry and media-savvy supporters -- long-stymied in their efforts to have the investigation reopened -- quickly rallied online support.

On April 29, Zhang Jie, lawyer to Zhu and her family, posted this tweet to Sina Weibo (it has subsequently been deleted):

“In traditional Chinese culture we not only say ‘the same rules apply to everyone even if he is a prince,’ but we also say ‘senior officials have the privilege of avoiding criminal penalties.’ This kind of contradiction appears in the Zhu Ling case. We want to capture the murderer and convict her (or him) of the crime, but the key fact of this case is that when oral testimony is needed, senior officials have the privilege to avoid it; after the prince breaks the law, the fact is there isn’t enough evidence to prove that he violated the law. These unspoken rules for protecting officials have existed in China for thousands of years, and we are challenging them.”

That challenge was soon met by Sina Weibo’s censors, who -- over the past 10 days -- became progressively more aggressive in managing, and censoring, the conversation about Zhu Ling. It’s impossible to know for certain whether this was proactive censorship that anticipated government orders or whether it was implemented at the explicit direction of the authorities. But from the standpoint of Sina Weibo’s users, the government appeared to be involved.

Among the earliest actions was a highly unusual censorship decision directed at People’s Daily, the official mouthpiece of the Chinese Communist Party. On April 26, the paper’s official Sina Weibo microblogging account tweeted, as translated by the blog Offbeat China: “Zhu Ling is 40 years old now, completely paralyzed, almost blind and with the intelligence of a 6-year-old. What exactly happened 19 years ago? Who was behind the poisoning?”

Many of China’s microbloggers took this as an explicit endorsement by the party of a new investigation, reposting the tweet 70,000 times and generating 30,000 comments, according to microbloggers who saw the comments and reposts.

Then the tweet was deleted by Sina’s censors, along with tweets that quoted it, posted screen grabs or reposted it outright. About the same time, People’s Daily deleted its special online page devoted to Zhu Ling coverage. So, either People’s Daily or somebody above it decided that the paper didn’t need to devote any additional coverage to an issue that was becoming increasingly critical of the party.

On April 29, a microblogger in Jiangsu province summarized the sense of foreboding the censorship of People’s Daily had brought:

“A powerful force has decided that microblogging related to Zhu Ling has become worthy of censoring and controlling, including those tweets written by celebrities and the People’s Daily. The force is so powerful it can withstand millions of microbloggers in pursuit of justice.”

Still, the momentum behind a reopening of the investigation persisted, and the pace of posts on Sina Weibo increased. On April 30, Zhu Ling was the top trending topic on the service for much of the morning and afternoon; then, at some point midafternoon, the name simply disappeared from the list. Searches still picked up posts including Zhu’s name, but someone seemingly decided it would be best for the name to appear a little less popular (though Sun Wei’s name continued to trend). Perhaps the hope was that such a move would slow the pace of posts. It didn’t.

Nothing the censors did worked. By late last week, users were posting tweets every few seconds, demanding justice and a reopened investigation. On Friday, Sina Weibo began deleting Zhu posts in earnest and preventing users from obtaining search results when they put “Zhu Ling,” “Sun Wei” and “thallium” into the service’s search bar. Sina Weibo’s users, accustomed to this kind of censorship, quickly invented English language hashtags and other means to circumvent the search ban.

More critically, several of the site’s celebrity users began posting on Zhu as well, thereby raising the profile of the case even further. Saturday night, Wang Ran, the chief executive officer of China eCapital, posted an extraordinary tweet telling his almost 3 million followers that the increased level of censorship proved the rumors of high-level intervention on behalf of the poisoner of Zhu Ling (his modification of her name allowed his tweet to get through the censors): “At first, the speculation surrounding Zhu L was just speculation. We kept emphasizing the need for evidence, kept emphasizing the criminal investigative process, kept emphasizing the legal principle of presumed innocence. We only appealed for an investigation, so that delayed justice would be found for the victim and family members. But then as of yesterday it was no longer possible to search for her. The victim has become a sensitive topic. As a result we can finally confirm that the basic rumors were true.”

For a Chinese government determined to corral public opinion in its favor, the failed attempt to shut down the Zhu debate is nothing short of a spectacular public-relations failure. On Friday, an enterprising netizen turned to the U.S. government and posted a petition to the White House website. It asks that the U.S. government “investigate and deport” Sun and pointedly takes note of her “family’s powerful political connections.” As of Monday afternoon in China, the petition exceeded the 100,000 signatures necessary for an official response from the White House. In all likelihood, that response will be diplomatic and noncommittal.

But even if there’s no response at all, the effort appears to have made an impression on somebody in authority in China. Late Monday afternoon, not long after the petition crossed the 100,000 signature threshold, Sina Weibo lifted its ban on related searches. Almost instantly, “Zhu Ling” was the top trending topic on Sina Weibo, followed by “White House.” This is a stunning, face-losing turnabout that’s unlikely to restore confidence in Chinese rule of law any time soon. But it just may result both in justice for Zhu and in providing China’s government with a powerful lesson in how effective its citizens have become at circumventing institutions that were designed to be infallible.

(Adam Minter, the Shanghai correspondent for the World View blog, is writing “Junkyard Planet,” a book on the global recycling industry. The opinions expressed are his own.)

To contact the author of this article: Adam Minter at ShanghaiScrap@gmail.com.

To contact the editor responsible for this article: Zara Kessler at zkessler@bloomberg.net.