NEW TERRITORIES detectives last night arrested two youths in connection with the death of a 13-year-old boy in what appears to be a kidnapping gone wrong. Hung Siu-chung's body was found in the pump house on the roof of a building in a Sha Tin public housing estate yesterday afternoon. Sha Tin Assistant District Commander (Crime) Superintendent Tam Tak-wing said there were no apparent injuries on the body. However, the boy's hands and legs were tied with wire and there was another piece of wire around his neck. His eyes and mouth were covered with adhesive tape. A post mortem examination will be carried out today. Siu-chung was the younger son of a senior fireman and lived with his parents in the Fire Services Department married quarters at Wo Che, within walking distance of Wo Che Estate where his body was found. The youths, both about 17, were being held this morning while detectives investigated a letter, made up of characters cut from magazines and newspapers, and placed in the Hung family's letter box. The family received a telephone call at 10 am telling them to go to the letter box to get a letter. The letter allegedly informed the family that Siu-chung ''is in our hands'' and demanded a ransom of $500,000. It said the ransom should be paid at a location somewhere outside the quarters. A report was made to the police but before there was any further development the body was discovered at 1.30 pm by a 41-year-old cleaning woman. The Form 2 student of a secondary school in Ma On Shan had joined his mother for yum cha at a restaurant on Sunday but after lunch he met a friend who invited him to a ball game. The family had not seen him since. A team of detectives last night went to a flat occupied by another fire officer in the same building and took (Cont'd from Page 1) away boxes of magazines and newspapers. An ambush was subsequently laid near the entrance to Ocean Terminal and the two youths were detained. Police are investigating access to the pump room as the iron gate to the rooftop should have been locked at all times. However, it is known that children in housing estates deliberately break locks and obtain keys for them so that they can use the rooftops as their own domain without alerting caretakers. Residents complained that Siu-chung's death was the most serious incident in a recent spate of youth crimes in the area. One 47-year-old housewife said the problem was getting so bad she was anxious about the safety of her two daughters and son, aged between 11 and 20. ''Many teenagers in school uniform gather on the staircase outside my flat to have lunch or smoke.'' She said young gangsters had robbed morning walkers and other teenagers on cycle tracks and in the city's gardens. Police appealed to anyone who might have seen someone acting suspiciously on the rooftop of Chi Wo House between 2 pm on Monday and noon yesterday to telephone 694-6308.