Police yesterday vowed not to give up the search for a missing off-duty officer, as rescuers went into a fourth day hunting for him in Sai Kung Country Park. Deputy district commander of Wong Tai Sin and acting Chief Superintendent Leung Man-kon, who made the pledge yesterday, said officers had so far searched 80 per cent of the 7,477-hectare park. Constable Ting Li-wah, 45, had a seven-minute conversation with 999 hotline staff and shouted for help before he lost contact on Sunday. Constable Ting, who sounded exhausted, told his colleagues that he went hiking alone for two hours in Pak Tam Chung. It is understood that the search operation yesterday focused in Tai Tan in the northern part of the park, where the officer was believed to have made the call. Rescuers tried to track down Constable Ting by testing the reception level with their mobile phones. The police also started to search routes less frequently used by hikers; some not marked on a map. Mr Leung said: 'We are still optimistic about finding the man alive because intermittent rain provided water for him. 'No deadline has been set; we'll keep looking.' But another officer said the chance of finding the constable alive was fading as he had been missing for more than 72 hours. 'What we can do is to pray for his safety,' the officer said. Deputy manager (operations) Raymond Yum Yat-keung of the Government Flying Service said a thermal imaging detector aboard a helicopter was used to help the search on Monday and Tuesday nights. 'We have searched most of the areas police requested but found no trace of the missing officer.' About 200 officers from Police Tactical Unit carrying maps and walkie-talkies divided into teams and searched not only along the paths and trails in the hills of the country park but also went into bushes to check. One policeman suffered heatstroke and was sent to hospital by helicopter. Yesterday's search included about 70 mountain rescue officers from the Civil Aid Service, 10 park wardens from the Agriculture, Fisheries and Conservation Department and 20 firefighters. At 3.45pm, Constable Ting's brother and some friends arrived at the search team base. They looked grieved after a brief talk with a police officer. The search was still continuing last night.