Hong Kong triads in the spotlight after Yuen Long attack: Who are they?

The issue of Hong Kong's triads has recently resurfaced following an incident last Sunday in which a mob of men armed with sticks and batons beat up commuters and passers-by in a metro station in the northern district of Yuen Long. Police have since arrested 12 people over the attack, a number with links to organised crime gangs. Here's a look at who these triads are.