When Jennifer Chen traveled back to her hometown in central China last winter for Lunar New Year, she thought little about Twitter.
She had around 100 followers on an account she believed to be anonymous.
While living in China, she retweeted news and videos, and occasionally made comments censored on Chinese platforms, like voicing her support for Hong Kong’s protesters and her solidarity with minorities who have been interned.
It wasn’t much, but it was enough for the authorities to go after her.
The police knocked on her parents’ door when she was visiting.
She said they had summoned her to the station, questioned her and then commanded her to delete her Twitter posts and account.
They continued to track her when she went overseas to study, calling her and her mother to ask if Ms. Chen had recently visited any human rights websites.
The Chinese government, which has built an extensive digital infrastructure and security apparatus to control dissent on its own platforms, is going to even greater lengths to extend its internet dragnet to unmask and silence those who criticize the country on Twitter, Facebook and other international social media.
These new investigations, targeting sites blocked inside China, are relying on sophisticated technological methods to expand the reach of Chinese authorities and the list of targets, according to a New York Times examination of government procurement documents and legal records, as well as interviews with one government contractor and six people pressured by the police.
To hunt people, security forces use advanced investigation software, public records and databases to find all their personal information and international social media presence.
The operations sometimes target those living beyond China’s borders.
Police officers are pursuing dissidents and minor critics like Ms. Chen, as well as Chinese people living overseas and even citizens of other nations.
The digital manhunt represents the punitive side of the government’s vast campaign to counter negative portrayals of China.
In recent years, the Communist Party has raised bot armies, deployed diplomats and marshaled influencers to push its narratives and drown out criticism. The police have taken it a step further, hounding and silencing those who dare to talk back.
With growing frequency, the authorities are harassing critics both inside and outside China, as well as threatening relatives, in an effort to get them to delete content deemed criminal.
One video recording, provided by a Chinese student living in Australia, showed how the police in her hometown had summoned her father, called her with his phone and pushed her to remove her Twitter account.
The new tactics raise questions about the spread of powerful investigative software and bustling data markets that can make it easy to track even the most cautious social media user on international platforms.
U.S. regulators have repeatedly blocked Chinese deals to acquire American technology companies over the access they provide to personal data.
They have done much less to control the widespread availability of online services that offer location data, social media records and personal information.
For Chinese security forces, the effort is a daring expansion of a remit that previously focused on Chinese platforms and the best-known overseas dissidents.
Now, violations as simple as a post of a critical article on Twitter — or in the case of 23-year-old Ms. Chen, quoting, “I stand with Hong Kong” — can bring swift repercussions.
Actions against people for speaking out on Twitter and Facebook have increased in China since 2019, according to an online database aggregating them.
The database, compiled by an anonymous activist, records cases based on publicly available verdicts, police notices and news reports, although information is limited in China.
“The net has definitely been cast wider overseas during the past year or so,” said Yaxue Cao, editor of ChinaChange.org, a website that covers civil society and human rights.
The goal is to encourage already widespread self-censorship among Chinese people on global social media, she said, likening the purging of critics to an overactive lawn mower.
“They cut down the things that look spindly and tall — the most outspoken,” she said. “Then they look around, the taller pieces of grass no longer cover the lower ones. They say, ‘Oh these are problematic too, let’s mow them down again.’”
Chinese security authorities are bringing new technical expertise and funding to the process, according to publicly available procurement documents, police manuals and the government contractor, who is working on overseas internet investigations.
In 2020, when the police in the western province of Gansu sought companies to help monitor international social media, they laid out a grading system.
One criterion included a company’s ability to analyze Twitter accounts, including tweets and lists of followers.
The police in Shanghai offered $1,500 to a technology firm for each investigation into an overseas account, according to a May procurement document.
Such work often begins with a single tweet or Facebook post that has attracted official attention, according to the contractor, who declined to be named because he was not approved to speak publicly about the work.
A specialist in tracking people living in the United States, he said he used voter registries, driver’s license records and hacked databases on the dark web to pinpoint the people behind the posts. Personal photos posted online can be used to infer addresses and friends.
A Chinese police manual and examination for online security professionals detailed and ranked the types of speech crimes that investigators seek out, labeling them with a one, two or three depending on the severity of the violation.
One denotes criticism of top leadership or plans to politically organize or protest; two includes the promotion of liberal ideology and attacks on the government; and three, the least urgent, refers to content ranging from libel to pornography.
The manual specifically called for monitoring activity on foreign websites.
The contractor said he used the rankings to classify infractions on dossiers he submitted to his bosses in China’s security apparatus.
In a sample document reviewed by The Times, he listed key details about each person he looked into, including personal and career information and professional and family connections to China, as well as a statistical analysis of the reach of the person’s account.
His approach was corroborated by procurement documents and guides for online security workers.
Over the past year, he said, he had been assigned to investigate a mix of Chinese undergraduates studying in the United States, a Chinese American policy analyst who is a U.S. citizen and journalists who previously worked in China.
Those caught up in the dragnet are often baffled at how the authorities linked them to anonymous social media accounts on international platforms.
The Chinese student in Australia, who provided the video recording from her police questioning, recalled the terror she had felt when she first received a call from her father in China in spring 2020.
The police told him to go to a local station over a parody account she had created to mock China’s leader, Xi Jinping. She declined to be named over concerns about reprisals.
In an audio recording she also provided, the police told her via her father’s phone that they knew her account was being used from Australia.
Her distraught father instructed her to listen to the police.
Three weeks later, they summoned him again. This time, calling her via video chat, they told her to report to the station when she returned to China and asked how much longer her Australian visa was valid.
Fearful, she denied owning the Twitter account but filmed the call and kept the account up. A few months later, Twitter suspended it.
After an inquiry from The Times, Twitter restored the account. A Twitter spokeswoman said it had been taken down in error.
Consequences can be steep. When a Chinese student living in Taiwan criticized China this year, he said, both of his parents disappeared for 10 days.
His social media accounts within China were also immediately shut down.
The student, who declined to be named out of fear of further reprisals, said he still did not know what had happened to his parents.
He doesn’t dare to ask because they told him that local security forces were monitoring them.
“Those who live abroad are also very scared,” said Eric Liu, a censorship analyst at China Digital Times, a website that monitors Chinese internet controls.
He said that Chinese users on Twitter were becoming increasingly careful, and that many set their accounts to private mode out of fear.
Mr. Liu’s account is public, but he screens new followers, looking for Chinese security officials who might be watching him.
For Ms. Chen, the police harassment has continued even after she moved to Europe this fall for graduate school.
She has struggled with feelings of shame and powerlessness as she has weighed the importance of expressing her political views against the risks that now entails.
It has driven a rift in her relationship with her mother, who was adamant that she change her ways.
Ms. Chen said that as long as she held a Chinese passport she would worry about her safety.
As a young person with little work experience and less influence, she said it was frustrating to have her voice taken away:
“I feel weak, like there’s no way for me to show my strength, no way to do something for others.”
Even so, she said she would continue to post, albeit with more caution.
“Even though it is still dangerous, I have to move forward step by step,” she said. “I can’t just keep censoring myself. I have to stop cowering.”