Four males were injured when a fight between rival factions of the Wo Shing Wo triad society escalated into a brutal attack after a seven-seater vehicle rammed a car on the Tolo Highway in Tai Po yesterday. The four injured males, two of whom were aged 16 and 17, were suspected triad members who were chopped shortly after 4am when five men armed with knives jumped out of the seven-seater, broke the windows of the car and attacked them, police said. The seven-seater was stolen, officers said. Since one of the vehicle's front tyres had burst, the attackers stopped a delivery van and ordered its driver to get out before escaping. 'After driving for about 800 metres, the delivery van crashed. The assailants were forced to flee on foot,' a police source said. The four were taken to Prince of Wales and Alice Ho Miu Ling Nethersole hospitals. The driver of the car, 22, was in critical condition. Two others, aged 16 and 19, were in stable condition and the fourth victim, 17, was discharged after treatment. Investigations showed that rival factions of the society were fighting for control of rural areas along roads that lead to a planned border checkpoint, expected to open in 2018, that will connect Heung Yuen Wai in the New Territories with Liantang in Shenzhen. 'The turf war between the two factions began more than six months ago after the Tsuen Wan-based gang tried to extend its influence in Sheung Shui,' one police source said. The source said the rival factions' previous clashes involved fist-fights and no knives were used. Another police source said: 'As there will be development projects along connecting roads to the new border crossing, triads will explore opportunities and use different forms and versions of illegal activities to make money.' This source said the illegal activities may involve buying land from villagers before developers and the construction of private housing estates, or triads may build car parks and godowns since there would be big demand in the future. Police described the attack as brutal and serious. It occurred about five hours after a suspected triad colleague nicknamed 'Ho Yin' was knocked down by a Mitsubishi Galant in an apparent ambush outside a pub in San Hong Street, Sheung Shui, at 11.15pm on Monday. A man of heavy build jumped out of the car and tried to attack Ho with a 45cm knife, but he fled in the Galant as Ho's associates rushed out from the pub, police said. The grey Galant sped away before police arrived, and the injured man was treated at North District Hospital. At about 5am, a car suspected to be linked to one of the two attacks was found torched outside Po Sheung Tsuen off Jockey Club Road in Sheung Shui. Police said it had also been reported stolen. Detectives from the anti-triad squad of New Territories North region have consolidated the investigation of the two attacks and arson case. The government has earmarked HK$9.6 billion to build the proposed checkpoint, which will be the city's sixth. A 10km trunk road linking the checkpoint with the Tolo Highway will improve access to development areas in Ping Che and Ta Kwu Ling. Shenzhen expects the checkpoint to handle about 28,000 vehicles a day. Two teenagers, aged 14 and 15, were arrested last night in Tin Sum, in connection with the attack on the Tolo Highway. There were 575 triad-related crimes in the first three months of this year, four cases more than in the same period of last year.