Following the recent acid attacks, police and two district councils have sent more than 1,000 letters to owners' corporations and building management companies, concerned about large crowds expected to gather in open areas over the holidays. The move came as police yesterday issued a HK$300,000 reward for information that helps solve the acid attack in Causeway Bay on December 12, in which a plastic bottle containing corrosive fluid was thrown from a building. It is the fifth cash reward offered for help in solving the series of acid attacks over the past year; the total amount of rewards on offer has now reached HK$1.5 million. Police expect the number of people gathering in Causeway Bay, Tsim Sha Tsui and Central on December 24 and 31 to be no fewer than last year, at more than 200,000 and 400,000 on the respective dates. Senior Superintendent Paul Lau, acting district commander of Yau Tsim District, said: 'Relying on the police alone [to prevent further acid attacks] is not enough. It is also important to rely on building management to pay more attention.' The letters sent to owners' corporations of buildings in Wan Chai and Yau Tsim Mong districts called on residents to report suspicious behaviour. Emergency paths would be set up where large crowds are expected, Lau said, adding that the planned deployment of 1,900 police officers on the two nights would be sufficient. A senior police manager said detectives would patrol the roofs of old buildings in Mong Kok, Sham Shui Po and Causeway Bay during the Christmas and New Year holidays to deter anyone from trying to throw acid in those crowded areas. Police officers will stop people without a proper purpose from going up to the roofs. At about 6.15pm yesterday, a plastic bottle usually used for medicinal alcohol was thrown from Tip Chui House on Butterfly Estate, Tuen Mun. No one was injured. The bottle, which contained a colourless liquid, landed on the ground near a building entrance where a safety net had not been installed. A net was installed by the Housing Department at the other entrance after a bottle of acid was thrown from the building at about the same time on Friday, injuring a man and his eight-year-old son.