Criminals may have launched the bloody assaults on rail passengers, but the incident is more likely to inflame than quell the anger at authorities

We are very unlikely to find out who arranged for thugs to rampage through a Hong Kong train station, armed with wooden sticks and metal rods, hospitalising 45 peaceful passengers. Yet if their approach was indiscriminate, their target was clear – protesters returning from an anti-government march – and their purpose equally plain: intimidation.

A pro-Beijing legislator was seen shaking the hands of the white-clad thugs at Yuen Long and has portrayed the men as local residents “defending their homes”. Carrie Lam, the region’s chief executive, condemned Sunday night’s violence – but spent more time criticising protesters who had surrounded Beijing’s liaison office and defaced its sign. Many in the protest movement disagree with such tactics. But political attacks on property can hardly be compared to a vicious assault that broke bones and left one man in critical condition.

There is a long history of thugs handling political business on the Chinese mainland – where local officials often outsource forced demolitions, for example – and in Hong Kong too. Back in 1993, the then chief of China’s Public Security Bureau said explicitly of triads that “as long as these people are patriotic, as long as they are concerned with Hong Kong’s prosperity and stability, we should unite with them”. In 2014, triads attacked pro-democracy demonstrators in the later stages of the umbrella movement.

So far six men have been arrested for unlawful assembly in connection with the Yuen Long violence, and police sources have said there are gang links. But the real question is not who they are, but why they were able to perpetrate their vicious attack.

Harsh policing in 2014 and at the start of these protests, including the use of teargas and rubber bullets, had already battered the hard-won authority of the police with residents. Now people want to know why, in a region with more than twice as many police per capita as England, it took so long for officers to reach the scene of the violence. While police claim they were tied up by the demonstration, witnesses say officers failed to arrest thugs even when they did reach the station. The assailants seemed so confident of impunity that some of those filmed beating people were not even masked.

Ms Lam and Hong Kong’s police chief have angrily denied allegations of collusion, with the chief executive calling them insulting. If Hong Kong police are to retain any credibility whatsoever, they must pursue Sunday’s attackers every bit as assiduously as they have pro-democracy activists, while the government must launch fully independent investigations of both the policing of demonstrations and the tardy response to the Yuen Long attacks, as protesters have demanded. Don’t hold your breath.

But a refusal by authorities to take this chilling attack seriously will guarantee that mistrust and antagonism continues to spiral. (Some worry that the assaults were intended to goad protesters into violence themselves.) While some will no doubt heed the thugs and stay away from future demonstrations, others appear more determined than ever to take to the streets. There are already calls for a rally at Yuen Long. And the protests, now in their seventh week, will burn on instead of burning out.