The person who threw bottles of acid into Temple Street night market on Saturday - just 30 minutes after a team of plain-clothes police patrolled the area - was probably the person responsible for four other acid attacks on busy pedestrian areas, an investigating officer said yesterday. Because of common factors, police suspect the attack, in which 30 people were injured, is the work of the person who carried out four attacks in Mong Kok and Causeway Bay beginning in December 2008. In each case two bottles of acid were used, he said, with other similarities including 'the targeting of pedestrian zones, choice of old buildings with lax security, and the use of the same kind of corrosive acid'. Police had identified the site at the junction of Nanking and Temple streets in Yau Ma Tei as a high-risk area for such an attack. The officer said the perpetrator appeared to have investigated the site and knew how to evade police and residents. 'It was a calculated act.' In Saturday's attack, the bottles of acid were thrown from the roof of 1D Nanking Street at about 9.30pm, half an hour after a team of more than 10 plain-clothes officers had left the site. Residents blocked the exits of buildings near the scene before police arrived, including 1D Nanking Street. A man was arrested at the scene about three hours later but police said he was not linked to the attack. An investigator said residents guarded the front exit but the attacker escaped before they arrived or slipped out the back. Since the attack officers have been sent to rooftop observation posts in Nanking and Temple streets every day to carry out surveillance. Officers had also stepped up patrols and surveillance in high-risk areas in the district before Saturday. Police have asked shopkeepers and residents to step up security in their buildings and to contact police if they see anyone acting suspiciously. Superintendent Joseph Yeung Chi-choi of Kowloon West regional crime unit said police were considering offering a HK$300,000 reward over the latest incident. Police have offered seven cash rewards totalling HK$1.7 million for help in solving the series of acid attacks. In all there have been eight attacks in urban areas and three in the New Territories. Six of the incidents took place last year in Sham Shui Po, Tuen Mun and Tin Shui Wai. Another senior police officer said district commanders had been instructed to identify high-risk areas, including pedestrian zones.