Hong Kong police on Thursday arrested six men linked to the Yuen Long attack on July 21 last year, bringing the total number detained to 43.
The force said officers had identified more suspects in the case, but some of them had already fled Hong Kong.
The attack was a turning point in the social unrest that gripped the city for almost a year since June last year. The police response to the attack prompted a public outcry, with officers accused of colluding with the aggressors by arriving late to the scene, an accusation dismissed by then police commissioner Stephen Lo Wai-chung.
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Senior Superintendent Chan Tin-chu, of the New Territories North headquarters' crime squad, said the six men, aged between 32 and 57, were held for rioting and conspiracy to wound with intent. Some of them allegedly had triad backgrounds.
But Chan refused to detail what roles the suspects had played, saying only that they were involved in violent attacks at Yuen Long railway station, areas nearby and Nam Pin Wai in the district.
“As to why the arrests took so much time, the case involved a lot of people. We need time to handle massive amounts of documents and videos. Many citizens have also provided us with new clips only recently,” Chan said.
He added the investigation had never eased up and his team had met often with the Department of Justice to discuss the matter.
Three of the six suspects are unemployed, one is a property agent, one a driver and the other an auto technician.
“We are looking into more individuals [linked to the case]. We have their identities, but unfortunately some have already fled. We have contacted the relevant law enforcement agencies overseas for assistance,” Chan added, appealing for members of the public to provide more details where available.
Police had also been criticised for taking 35 minutes to send officers to Yuen Long MTR station on the night of the incident, in which about 100 armed men dressed in white T-shirts attacked extradition bill protesters and other passengers.
Seven minutes after the attack, two officers arrived on the scene but backed off and called for help after deciding they did not have the equipment to deal with the armed crowd.
The assailants had left by the time police reinforcements arrived at 11.20pm, with 45 people injured at the scene.
A total of 37 people – some with links to triads – had earlier been arrested for their alleged roles in the attack, seven of whom have been charged with rioting and conspiracy to wound with intent. Police said they were continuing to collect evidence.
Then police chief Lo admitted that officers arrived 35 minutes after receiving the first report of attacks on protesters at the station, but denied any collusion with triads over the mob violence.
He said at the time the force’s capacity had been stretched across Hong Kong Island dealing with protests.
Secretary for Security John Lee Ka-chiu told the Legislative Council last November that police had received information about activities in Yuen Long beforehand but it was inconsistent.
Upon assessment, police deployed uniformed and plain-clothed officers in Yuen Long to work in vehicles and on the ground, and activated an operation command centre at 6pm on the day for monitoring and responding to the situation, Lee said.
The emergency services hotline in the northern Hong Kong command and control centre was jammed with 24,000 calls in three hours that night – an average rate of two per second – meaning they would each have had to deal with a call every five seconds to take them all. The centre usually handles an average of 2,500 calls a day.
Lee said “police-triad collusion” was an “extremely unfounded and malicious allegation”.
“I must point out seriously that police always stand against lawbreakers, especially triads, and definitely fight them comprehensively,” Lee told lawmakers.
Earlier this year, eight people hurt in the Yuen Long mob attack sued the police chief for compensation.