A 36-year-old mainland woman was arrested in a suspected arson case in Ngau Tau Kok yesterday. At about 4am, 600 residents of a building were evacuated when a flat in Choi Ha Estate caught fire. Three people, a woman, 63, her son, 36, and his girlfriend, 39, were inside the flat. The women were overcome by smoke and the man suffered slight face injuries. It was suspected that paint thinner was used to set the flat on fire.