KUALA LANGAT: A 12-year-old boy today recounted how he watched as his grandparents and brothers were killed before their wooden house in Tanjung Sepat here was set on fire. "Grandma, grandpa, little brother, my second brother all dead ... except me. Father went to work ... not back yet. "Thief killed them and burned the house. All dead! They were all killed by the thief ... with a big knife," the sole survivor of the incident said in a video clip that went viral after the incident. Police have confirmed that the video of the boy recounting the incident in Mandarin, is part of their investigations. The boy, whose identity has been withheld by police, said he hid in the toilet at the time of the incident, which occurred in the wee hours today. The incident in which four family members including a three-year-old toddler were tragically killed, has been classified as murder after the Fire and Rescue Department and police investigators discovered elements of foul play. The remains of Chia Tee Nang, 73, his wife Lim Mai Shak, 68, and their grandsons Chia Yuan Bin, three, and Ryan Chia Zeng, nine, were discovered in various parts of the burnt home following the fire. The father of both boys, a mute, and other relatives were distraught and seen weeping at the scene of the incident. Kuala Langat district police chief Supt Azizan Tukiman confirmed that the sole survivor of the incident escaped through a door near the bathroom of the house. He said the boy did not suffer any injuries but was admitted to the Banting Hospital for observation, as he was in a traumatic state following the incident. "We will give him time to record his statement," Azizan said. Selangor Criminal Investigation Department (CID) chief SAC Fadzil Ahmat said initial investigations by the Fire and Rescue Department following the 6am fire today revealed elements of arson. "The Selangor Crime Scene Investigation (CSI) team also found wounds on three of the victims who are believed to have been stabbed with sharp objects. "The victims were believed to have been stabbed before their bodies were set on fire," he said. Fadzil said police are investigating the crime from all angles including claims that the victims were robbed before being killed, possibly as an act of revenge. It is learnt that investigators believe the family car was used to set the house on flames as traces of fire accelerants were discovered around the fuel tank. The Fire and Rescue Department, which deployed two fire engines, an ambulance and 15 firemen from the Sepang and KLIA fire stations to the scene upon receiving a call at 6.08am, took about 40 minutes to put out the blaze before recovering the bodies between 6.43am to 6.58am.