Police warned villagers against forming vigilante groups in reaction to the weekend murder of a Yuen Long couple. Detectives said the tragedy, in which To Kap-shing, 63, and his 59-year-old wife, Pang Siu-mui, were killed with a 30-centimetre chopper in their Shui Pin Wai home, should serve as a warning to other residents. The couple could have been alive but for their habit of sleeping with doors and windows open, they said. Fears have been ignited by a sharp rise in the number of violent - and armed - illegal immigrants from China. But Senior Superintendent Andy Tsang Wai-hung of New Territories North cautioned strongly against vigilante groups: 'It can lead to legal problems if they target an innocent stranger. 'And secondly, if dangerous IIs know they could meet local resistance, they will come prepared,' he said. 'In this particular case had the old man shut his door, this wouldn't have happened. It is a real tragedy.' The number of arrested illegal immigrants armed and from northern provinces has risen from 4,675 in the first six months of last year to 4,925 in the first six months of this year. Yuen Long District Board member Tang Hing-ip said: 'Villagers are used to keeping their doors open because all the neighbours are from one family.' Mr Tsang said the increase in the number of northerners was fuelled by a crime crackdown in Shenzhen, driving northerners out of the area, and rumours of a post-1997 amnesty for mainlanders who reach the territory. A mainland illegal immigrant, Lau Sze-kwan, 22, from Heilongjiang, was last night being questioned by police in connection with the killings.