The announcement was made by Deputy Prime Minister Surapong Tovichakchaikul, who is in charge of Capo, and referred to the attack that left at least 36 people wounded. One one of them, Prakong Chuchan, 46, a resident of Nakhon Si Thammarat, died on Saturday morning.
Mr Surapong said that according to evidence collected from the crime scene, including the footage of security surveillance cameras in the area and the questioning of witnesses by police, it was found that the grenade that went off was not thrown from any of the high buildings along Banthat Thong road.
Pol Maj Gen Adul Narongsak, deputy chief of the Metropolitan Police, said it was not possible for the grenade to be thrown from a high spot like a building where an air gun and other articles were found because it would be obstructed by tree branches and electrical wiring.
Video clips examined by police showed two men acting in a suspicious manner. One of them, wearing a white hat, walked quickly from the tail of the marching column, dropped an object and ran to hide behind an iron box containing telephone circuit boards. After the grenade exploded, he and another man walked from the box to pick up fragments of the bomb and left without paying attention to the people who were injured and lying on the road.
Mr Surapong said police investigation of the blast at Banthat Thong road showed it was apparently created to harm demonstrators but not PDRC leaders.
"The government regrets the loss of life and injuries of innocent people and condemns those who were behind the incident," he said.