Police are treating as arson a fire which badly damaged four double-decker buses early yesterday. Fire crews and police rushed to a bus terminus in Tsz Wan Shan, east Kowloon, after one of the vehicles was spotted ablaze around 4.30am. The flames quickly spread to three other buses, burning one down to its frame. All four were owned by KMB. Wong Tai Sin fire station officer Leung Wai-fung said what appeared to be a flammable substance was found at the scene in Yuk Wah Street. Detectives from Wong Tai Sin district crime squad are investigating the case. No arrests have been made. Police said the fire started about three hours after the vehicles were parked. 'We believe the blaze started at the front of the bus, near its entrance,' said Leung, speaking of the first bus to catch fire. Thick smoke billowing from the burning vehicles forced the evacuation of about 100 residents from the nearby Man Po and Ka Wah buildings. A 68-year-old woman security guard in the Man Po Building who inhaled smoke was treated at Queen Elizabeth Hospital in Yau Ma Tei and later released. One resident said fumes from the fire wafted into his 17-th floor flat. 'When I looked down, I saw black smoke billowing from the burning vehicles,' he said. Five fire engines and one ambulance were sent to the scene. Firefighters took more than an hour to douse the flames. Police cordoned off the bus terminus as government chemists searched for evidence from the burnt-out vehicles yesterday morning. KMB has used the terminus as a night parking lot for years. A company spokeswoman said no guard was assigned to the lot, although security personnel visited such lots during nightly patrols. All four damaged buses served the same route - 3D - from Tsz Wan Shan to Kwun Tong's Yue Man Square. Backup vehicles were deployed on the route and operations were unaffected, the spokeswoman said. She said KMB had not received any threats before the attack.