Choy's case is emblematic of the constantly shifting rules in the city — the new reality that actions deemed reasonable one day could lead to prosecution the next.

Ticking a box

On a summer night in July 2019, a mob of about 70 men in white shirts stormed a Hong Kong subway station and began to beat commuters and protesters with iron bars and bamboo sticks.

Videos from the night show commuters screaming in subway cars in Yuen Long station, near the border with mainland China. The men appeared to target those dressed in black returning from pro-democracy demonstrations in another part of Hong Kong.

Later, the police said officers were busy with protests on Hong Kong island, some 30 kilometers (19 miles) away. None of the white-clad men was arrested that night, although dozens were arrested subsequently.

It was that pivotal event that public broadcaster Radio Television Hong Kong (RTHK) and freelance producer Choy wanted to investigate.

Several of the license plates, Choy found, were linked to village representatives, or local leaders.

RTHK's documentary offered more evidence that when the white-clad mob attacked people in Yuen Long's train station, they had some official support.

A violation or a search for truth?

After RTHK's 23-minute documentary, "Hong Kong Connection: 721 Who Owns the Truth," was released last year, it won praise and awards, including one last Wednesday from the Hong Kong Journalists Association (HKJA).

But on November 3, 2020, Choy was arrested on suspicion of violating the Road Traffic Ordinance.

Choy is believed to be the first journalist convicted of violating the ordinance, and the first person sentenced in connection with the Yuen Long attacks, according to her lawyer, Jonathan Man. Last week, police confirmed they arrested a reporter from state-owned pro-Beijing outlet Ta Kung Pao over the same charge in February.

After the verdict, Choy's eyes grew red as she stood, surrounded by cheering supporters and media, outside a court in West Kowloon where many pro-democracy activists have been prosecuted over the past year.

"I believe that investigative journalism is not a crime," she said. "My journalistic values will not be affected by this case."

Chris Yeung, the chair of the HKJA, called the verdict a "dark day" for Hong Kong journalism.

"Press freedom in Hong Kong is dying," he said. "It's a fine for all journalists."

To Lokman Tsui, an assistant professor at Chinese University Hong Kong's school of journalism and communication, said the case is a sign the government is moving the goal posts.

But the case set a precedent that looking up a license plate for journalistic reasons is no longer legal — another blow for freedom of information, Tsui said.

Tsui said it also appears to show that authorities are prepared to go after journalists who dig up things that made them look bad. "It's hard to not see this as an attack on truth," he added.

In a statement to CNN Business, the Hong Kong government said any arrest had "nothing to do with the political stance, background or occupation" of the person concerned.

Other blows

"Everyone in Hong Kong is self censoring," said Tsui said. "In the last couple of years, there has been a sustained attack — not just on press freedom, but on rights in general."

Several events in 2018, for example, had a particularly chilling effect on the media environment.

And media freedom advocates argue that the landscape has only gotten more hostile since last year's national security law came into effect.

While Carrie Lam, the city's leader, said after that law was passed that Hong Kong people should still be able to enjoy freedom of speech and press, the new rule was later used to bring charges against media mogul Jimmy Lai, the founder of the pro-democracy tabloid Apple Daily. As police raided his newspaper's offices, they charged him with organizing an unauthorized protest and colluding with foreign forces.

The national security-related charges against Lai are still pending, and the legislation has not yet been used against reporters. But Tsui, from Chinese University Hong Kong, said that could change in the future.

In a statement to CNN Business on Tuesday, the Hong Kong government said it is "firmly committed to protecting and respecting the freedom of the press, which is a fundamental right guaranteed by the Basic Law."

When asked whether it was likely journalists would be prosecuted under the national security law in the future, the government said that "law-abiding people will not unwittingly violate the law."

Pressure on journalists has continued to mount this year. In February, for example, Xia Baolong, the director of China's Hong Kong and Macao Affairs Office, called for Hong Kong's media to be run by "patriots."

Members of the media are feeling the squeeze, too. Last year, the HKJA found a third of journalists surveyed felt pressured by their seniors to drop or reduce reporting on Hong Kong independence. And Keith B. Richburg, director of the Journalism and Media Studies Center at the University of Hong Kong and the head of the city's Foreign Correspondents Club, said journalists had noted some sources didn't want to talk anymore.

Pressures on RTHK

In an emailed statement to CNN Business, RTHK said it does not comment on individual court cases or staffing issues. The broadcaster said that it suspended the BBC World Service after the National Radio and Television Administration announced that BBC World News was not allowed to continue within Chinese territory. Its programs must abide by the charter, the producers' guidelines and the laws of Hong Kong, the broadcaster added. "As stipulated in the Charter, RTHK is editorially independent."

As for Choy, the RTHK reporter: When asked by the media Tuesday whether Choy's verdict undermined investigative journalism, Lam said nobody is above the law.

"If the law today doesn't allow you to do certain things — and even though we respect freedom of speech and I recognize your profession — you cannot do it. We need to balance the interests of different parties."

Beijing's economic influence

There are concerns that mainland China's increased economic influence on Hong Kong could exert pressure on media outlets in the city.

Clement So, a Chinese University of Hong Kong professor who studies Hong Kong's media landscape, said in the past decade, a growing number of media organizations had mainland Chinese investment, something he believed could lead to self-censorship.

After prominent English-language paper the South China Morning Post (SCMP) was bought by Chinese tech giant Alibaba in 2015, for example, there was concern the paper's editorial freedom would be compromised. Critics have hyper-analyzed the publication for signs of Beijing's influence — but it has continued to report on topics that Chinese state media don't touch, such as the Hong Kong protests and human rights abuses in Xinjiang.

An SCMP journalist — who asked not to be named — said they and others in the newsroom felt an "instant panic," fearing the buyer could be a state-owned entity or pro-Beijing company.

In an internal email seen by CNN Business, however, SCMP chief executive Gary Liu said Alibaba's commitment to SCMP "remains unchanged," and the company "will not be responding publicly to these unsubstantiated rumors." CNN Business has reached out to Alibaba for comment.

The reports also provided a silver lining, though. To the employee, the widespread concern over SCMP's future showed the paper's value.

"At least we're having conversations now about why the SCMP is actually playing quite an important role and what that would mean if it was lost," the SCMP journalist said.

Follow the money

To journalist Ronson Chan, vice-chairman of the HKJA, all this points to an unmistakable conclusion: there's now a possible danger in being a journalist in Hong Kong.

"If you ask my heart, of course, I know being a journalist, especially working for a non-Beijing controlled media, must have some caution or possible danger," said Chan, who has worked across 11 media outlets, including at non-profit investigative news agency FactWire, where he worked with Choy.

Now an editor at non-profit pro-democracy news website Stand News, he says he wouldn't be shocked if he was arrested.

Part of the problem is a fundamental disagreement over the purpose of news. While Western journalism theory sees value in objectivity and holding authority to account, Chinese leaders see it has a "tool of political propaganda," he said.

Chan believes China's history of cracking down on dissidents and journalists tells him he should leave the city, before he is prosecuted. That's something he has discussed with his wife. "I don't know if it will become an evidence in my prosecution talking to you today," he said.

It wouldn't be the first time: interviews Apple Daily's Lai gave to international media were cited as evidence in his national security law cases.

But in the end, Chan wants to continue reporting in his city.

"Hong Kong is our home," said Chan. "If we left, the Hong Kong people have no news to read."

-- CNN's Eric Cheung contributed reporting from Hong Kong.