“The investigation has progressed a lot. Police believe this incident was carried out by a political group bent on violence. They are extremists,” Chakthip said.

He added that 61 out of more than 80 witnesses had been questioned by investigators and at least five provided “very useful” information.

Meanwhile, Army commander-in-chief General Chalermchai Sitthisart said hardline red-shirt leader Wuthipong “Ko Tee” Kotchathammakhun could be involved in the attack.

He added that Wuthipong, who has been in exile in Laos, often campaigned for violent causes through social media.

“Ko Tee is just one of the suspects,” said the Army chief, in his capacity as secretary-general of the National Council for Peace and Order.

But he declined to confirm speculation that pro-red shirt military officers were involved in planting the explosive device at the military-run hospital.