Thailand on Wednesday blamed a deadly blast at an apartment complex near Bangkok on the anti-government "Red Shirt" movement as investigators probed whether a bomb-maker caused the explosion accidentally. Authorities said the powerful blast, which killed at least three people and injured nine late Tuesday, was believed to be linked to political unrest. "It's clear that the bombers are Red Shirt people," Defence Minister General Prawit Wongsuwon told AFP. "As far as I know there were several bombs stored in that room, as you can see that it was very powerful. There are a lot of bombs kept in other places," he said. Police found three bodies and one severed limb, suggesting the final death toll could be four, said Chatree Charoencheewakul, head of the Emergency Medical Institute of Thailand. The blast - believed to have occurred in a room on the second floor of the apartment building in Nonthaburi Province north of Bangkok - shattered windows and damaged nearby buildings. "It was about 10 kilos (22 pounds) of explosive material," Lieutenant Colonel Kamthorn Ucharoen of the Explosive Ordnance Disposal and Identification Special Operation Division said. "We are checking what type of bomb it was but we found nitrate. The bombers were probably making a crude bomb inside room 202 and there was an accident and it exploded prematurely," he said. The explosion came hours after Thailand extended a state of emergency in Nonthaburi and three other provinces, including Bangkok, for three more months. The emergency laws were introduced in the capital in early April in response to mass anti-government rallies by the "Red Shirt" movement that ultimately left 91 people dead in clashes between demonstrators and the army.