Security cameras caught two men torching rubbish outside a Masterton supermarket on Saturday night that threatened the entire building with fire while cleaners were on duty inside. Masterton fire service station officer Mike Cornford said police and fire investigators are treating as arson the midnight blaze at Kuripuni New World supermarket,which filled the building with smoke as the people inside escaped to safety. It "could easily have become a very, very serious arson", Mr Cornford said. He said the blaze was started about midnight in cages of plastic wrap at the rear of the building. It damaged exterior cladding and spread to frames within the walls. Police and fire investigators are continuing inquiries into the blaze, he said. The arson at the supermarket continues a prolonged run of deliberately lit fires in the region over the past five weeks, and multiple nuisance fires set in skip bins and rubbish throughout the Masterton business district over the same time. Firefighters and police last week warned in the Times-Age that business owners must stack their refuse away from buildings or risk losing their building and property to an arsonist. "I don't think our warnings are sinking in. A skip bin was set alight at the rear of a Queen Street business just over a week ago that could have been very serious as well, but the bin is full of cardboard and back in exactly the same place," he said. "There will be a major fire if business owners don't take this seriously, without doubt."