The gangland-style assault on independent journalist Kevin Lau in Hong Kong is a bad omen for the city's continued freedoms and the prospects for promised free elections in 2017. Mr. Lau is recovering from his wounds, following the Feb. 26 attack, but the fear of a Chinese crackdown on the city remains very much in the air.

China made a promise of free elections when it took over Hong Kong from British rule in 1997, preserving British style courts and press freedoms along with limited voting rights for citizens. The city has remained one of the world's most prosperous, serving both as a gateway city to China's productive south and as an international center of banking and finance, ranking third in the world behind London and New York.

It also has a flourishing free press. Among the leading journals is the Chinese-language Ming Pao, where Kevin Lau formerly served as editor. The newspaper has reported on the mistreatment of dissidents in China and exposed corrupt business deals there.

No one has claimed responsibility for the life-threatening knife attack on Mr. Lau, but the background to the attack suggests a motive of revenge for the newspaper's reporting - and a warning to other newspapers that would expose corruption.

Last year under Mr. Lau's leadership Ming Pao helped an international team of investigative journalists connect the names of relatives of China's top leaders to secret offshore bank accounts in tax havens such as the British Virgin Islands and Samoa. Mr. Lau was fired from his editorship shortly before the articles detailing these links began to be published in January. Ming Pao did not publish the stories.

The Chinese government dismissed the reports. But it also prohibited and blocked their distribution in China.

Among the secret accounts exposed were ones belonging to the brother-in-law of President Xi Jinping and the son and son-in-law of former Premier Wen Jiabao.

Mr. Wen, already embarrassed by a New York Times report on his family's hidden riches in China, wrote a letter to a Ming Pao columnist denying knowledge of his relatives' financial activities. But a pattern of retaliation for embarrassing stories had already been established. The Times exposed his family's wealth in 2012. Shortly thereafter a Times correspondent in Beijing was forced to leave the country.

The attack on Mr. Lau was not an isolated event. The Wall Street Journal reports a spate of physical attacks in recent years on Hong Kong media critical of Communist China and its local friends. Many cases go unsolved because the perpetrators come and go from mainland China, beyond the reach of the Hong Kong authorities.

On March 9, thousands of protesters took to Hong Kong's streets to rally for press freedom. It represented the broad recognition that an unmuzzled press is the best guarantee that China will live up to its larger promise for individual freedom, including free elections in 2017.