Four youths were arrested last night after a Ma On Shan family laid an ambush because of suspicions items were being stolen from its flat. Family members switched off the lights and detained a 14-year-old girl, a classmate of their son's, who opened the flat doors with keys. Three other teenagers fled. Two youths and another girl, aged 14 or 15, were later arrested by police. The occupant's 14-year-old son told his father he had recently lost the keys to the flat at school. Police allegedly found a set of duplicate keys to the flat, on the 14th floor of Kam Keung House in Kam Ying Court, on the girl arrested there.