A surveillance camera photograph of a suspected sex attacker was the only clue police had to work with after a 12-year-old schoolgirl was indecently assaulted on the roof of an old building in Sham Shui Po. But after a two-month, around-the-clock manhunt that detectives described as like 'finding a needle in a haystack', investigators arrested a 24-year-old salesman in Wong Tai Sin yesterday. The alleged attack was on June 22 when the Primary Six pupil was going home in Sham Shui Po after school. The five-storey building where she lives with her family has no lift, security guard or closed-circuit TV. 'She was intercepted in the staircase, robbed of HK$20 and threatened with a knife before being dragged to the roof of the building,' a police officer said. 'The sex attacker lowered his trousers and underwear, demanded oral sex ... and tried to undress her. At this critical moment, her telephone rang when her mother called her. The phone ringing apparently shocked him and she pushed him away and ran off.' The terrified girl ran back home to seek help from her mother, who called police. The attacker fled before officers arrived. But he was captured by a surveillance camera on the office of a construction site next to the building, police said. Another officer said: 'To track his escape route, our colleagues had to check all closed-circuit television footage at shops and buildings along a one-kilometre stretch.' The suspect was tracked to Sham Shui Po MTR station, so officers collected the Octopus card data of commuters and viewed closed-circuit television tapes at MTR stations to identify the man and his movements. After analysing the data and footage, investigators concluded that he lived in Chuk Yuen (North) Estate in Wong Tai Sin. From Saturday, more than 10 crime-squad officers from Sham Shui Po police station lay in wait in the estate around the clock. The man was arrested at 8am yesterday when he left one of the estate's buildings. Officers seized a computer from his home for examination. Police were also investigating whether he was linked to other sex attacks in the city. Meanwhile, police arrested a suspected serial sex attacker in Ching Ho Estate, Sheung Shui yesterday. The 27-year-old man is alleged to have been involved in five recent cases of indecent assault on the estate. He was detained overnight.