Police are still ploughing through the task of investigating thousands of people as they search for suspects in four cases of acid throwing in Mong Kok and Yau Ma Tei going back to 2008. Officers identified those people after studying about 200 lots of closed-circuit television images and making inquiries. But despite these painstaking efforts - and the four rewards on offer worth HK$1.2 million - they have yet to make a breakthrough. "We have to identify them one by one, check their backgrounds and establish whether they are suspicious before removing them from our list," a police source said. "Up to now, we haven't identified all of them." Three of the attacks took place in the Sai Yeung Choi Street pedestrian zone between December 2008 and June 2009 and the fourth on Nanking Street in January 2010. A total of 131 passers-by were injured. Police check the CCTV footage for whether the same person or people appear at the different crime scenes. They have not ruled out the possibility that the same culprit was behind all four cases as the bottles and acid - a drain cleaner - were similar. Investigators also checked whether the owners and assistants of shops at the four locations were in dispute with anyone, and looked into hundreds of hardware shops across the city that sold drain acid. "In the beginning, we received some calls. But there has been no response in the past two years," another source said. In what could be the latest acid attack, a bottle of drain cleaner wrapped in a plastic bag dropped from a Sham Shui Po building on Wednesday night. It fell onto a parked vehicle and bounced to the ground, splashing acidic solvent on a South Asian passer-by, 43. He suffered minor burns. No one has been arrested. In the past four cases, no plastic bags were used.