There are networks of operatives and other people involved and the attack who had nothing to do with the insurgency in the far South, Prawit reportedly told a meeting of ministers working on a public relations plan to announce the Prayut Chan-o-cha government's achievements over its six months in office.
National police chief General Somyot Poompanmuang backed Prawit's comments, saying there was sufficient evidence against the masterminds of the attack but police were in no hurry to obtain court warrants for their arrest.
"Hurrying the case may result in the case being put at a disadvantage. Although we have obtained sufficient evidence, we are in no need to be in a hurry. We can wait until we are completely confident, before taking further action," he said.
But when pressed on whether there was sufficient evidence against the masterminds, Somyot said he could not answer that yet.
He also refused to say whether a group of politicians that travelled to Surat Thani, on the mainland across from Koh Samui, would be summoned for questioning. "That will be decided later by case investigators. I cannot answer that," he said.
Prayut had earlier called on the "South-based politicians in question" not to panic, saying that he had not named anyone and people not involved in the attack should not react or pre-emptively deny the allegation.
Police based in the far South are looking for another three vehicles seen in surveillance camera footage travelling with the silver Mazda truck used as the vehicular bomb.
They are a silver Honda Civic sedan, a white Mitsubishi Triton pickup truck and a four-door Isuzu D-Max pickup.
The three vehicles are believed to have returned to the strife-torn region and could possibly be made into car bombs for future attacks, a source said.
Yala police plan to summon a driver with a local administrative body that owned the Mazda used in the attack. He had reported it stolen.
Abdulrasah Dumedae has given inconsistent information on the theft of the vehicle in March and has not provided any information deemed useful by security authorities, according to a security source.
The court-approved summons for Abdulrasah is needed for further questioning after his seven days in military custody ended yesterday.
Three security guards employed by Central Festival yesterday underwent physical examinations at a civilian hospital to make sure they had not been abused physically while detained by the Army before they were to be handed over to police, said Maj-General Kuakool Innajak, the chief of the Surat Thani military district.
He said one of the three guards had suspect elements in his statement and police would continue questioning him, after their detention with the military ends next Tuesday.