China's shadowy triads have long been suspected of doing Beijing's dirty work.
Perhaps most commonly associated in the West with over-the-top action sequences in kung-fu films, the term “triad” refers to various sects of criminal gangs which often harbour intense rivalries with each other.
Hong Kong is a hub of triad activity, a legacy of the Communist era purge of organised crime from the mainland.
The ancient gangs have been associated with the suppression of protesters and troublemakers in both Hong Kong and parts of southern China in recent years, where they act as thugs for hire.
Sunday's attacks were carried out in plain view of security cameras, and with a mysterious lack of police presence.
The most prominent triad gangs in Hong Kong are 14K and Sun Yee On, and there have been claims that the Wo Sing Wo group are behind Sunday’s attack.
In 2014, triad thugs riding a motorcycle attacked the outspoken editor of a Hong Kong newspaper editor with a meat cleaver, leaving a six-inch wound on his back. Members of the Shui Fong triad, it has been claimed, carried out the attack in exchange for payments of one million Hong Kong dollars (£100,000) each.
THREAD: These unsettling #HongKong videos going viral speak for themselves. If you knew *nothing* about the #HongKongProtests you’d still know this is not right. Local reports say pro-gov’t triads beat anti-gov’t protestors - and anyone in between - at Yuen Long rail station. pic.twitter.com/qLFp76vUNj— Ramy Inocencio 英若明 (@RamyInocencio) July 21, 2019
In the same year, at the height of the anti-capitalist Occupy Movement protests in Hong Kong, suspected triad members beat protesters and destroyed their tents in the Mong Kok district, the gangs’ heartland.
Local authorities in mainland China have also been accused of paying triads to forcefully evict homeowners from their property.
In one case in 2011, a middle-aged woman who refused to move out died after her house was demolished by a gang while she was still inside. As far back as the 17th century, triad gang members were pawns in political struggles, including one attempt to overthrow the Qing dynasty and restore the preceding Ming dynasty.
When the Communist Party took power in China after the Second World War, vast numbers of triad gang members fled to Hong Kong.
Later, in the 1960s, the height of triad activity in Hong Kong, police suspected up to one in six people were members of roughly 60 different triad gangs.
More recently, drug trafficking has become a significant source of the triads’ income, along with extortion, money laundering, gambling and prostitution.
Gang members are often recruited in their late teens, and must take 36 oaths as part of their initiation ritual.
Initiates are warned they will be killed "killed by five thunderbolts" if they fail in their duties.