Octopus cards, the pieces of plastic used by millions of people every day to travel and shop, have become one of the police force's secret weapons in the fight against crime. The cards can give vital clues to the identity of culprits even when there are no witnesses or fingerprints at crime scenes. And when the system is integrated with the Shenzhen Tong smart card in Hong Kong's neighbour city, police may be able to track crooks on both sides of the border. 'Logically, this will work,' a veteran police officer said. 'But we will have to seek help from mainland public security officers because we cannot exercise jurisdiction on the mainland.' Every Octopus card contains a chip that stores its outstanding balance and transaction records. Each card has its own serial number for identification. Some are personalised, making them even easier to trace. Officers say the card is like a global positioning system because the data can tell when and where its holder uses it. As well as on the transport system, they can be used to buy meals, pay car park fees, buy drinks from vending machines, shop at supermarkets and convenience stores, and enter offices and schools through their security systems. Evidence such as a receipt for a convenience store purchase left behind at a crime scene can link a card to a suspect. Once such a link is confirmed police can use it as a clue to track and identify its holder even if it is not a personalised card. This was how they tracked down a suspect in a Causeway Bay acid attack who was arrested last month. Together with the card data, police use other tools such as closed-circuit television footage at convenience stores, supermarkets and MTR stations. Following the transaction records, officers go to the locations visited by the card holder, and then check security camera images to identify the card user. 'If footage is unable to capture the image of the user, police have to investigate the transaction records in order to find out the movements of its holder,' a senior police officer said. 'Officers then lie in wait at a number of the locations regularly visited by the card holder.' After identifying the right person, officers from the criminal intelligence bureau will tail the suspect day and night. This is how police followed a black paper bag left behind at the scene of an acid attack in Causeway Bay in December. They worked around the clock for about a month and arrested a man in January. In this case, officers from Hong Kong Island regional crime unit checked more than 50 CCTV tapes from Causeway Bay MTR station and identified a man carrying a similar bag who left the station before the attack. Following this lead, officers collected the Octopus card data of commuters who passed through that turnstile, matching times with footage, to identify the suspect. In another case, crime squad officers from Eastern police station used similar tactics to solve a case in November 2008 in which the body of a newborn baby girl wrapped in a plastic bag was found abandoned in a supermarket in North Point. Officers checked the transaction records of Octopus cards used for shopping at the supermarket around the time the baby's body was found. They investigated the card holders one by one and then arrested a 67-year-old male herbalist, Octopus cards owned by two prostitutes killed in To Kwa Wan in March 2008 and January last year also led police to arrest two men who stole the cards and used them to buy goods after the killings. Checking the card data is not free of charge. The officer said it was not cheap to get the printouts of the card's transaction records. 'Each printout contains about 50 transaction records and costs a few dollars. In one case, we may need to check and analyse a massive amount of transaction records. It's relatively expensive,' the officer said. Every Octopus card contains a chip that stores its outstanding balance and transaction records. Each card has its own serial number for identification. At present, there are more than 20 million Octopus cards in circulation in Hong Kong, which has a population of more than 7 million. 'Painstaking detective work and hard work by our officers are vital,' the veteran officer, who has been involved in criminal investigation for more than 20 years, said. 'Officers have to look with bare eyes to check tapes of surveillance cameras and analyse a massive collection of card data.' In relation to the three unsolved acid attacks in Mong Kok that injured more than 100 passers-by, officers have conducted door-to-door inquiries with 6,549 people, studied 184 sets of closed-circuit television footage and contacted all 584 hardware shops in the city that sell acid. So far, no one has been arrested. 'As a responsible corporate citizen, Octopus will, under normal circumstances, provide information to the police as required by law, such as search warrant and court orders,' a spokeswoman for the Octopus Card management company said. She said Octopus valued the importance of customer data privacy and protection, and fully complied with the relevant ordinances.