A bomb exploded at an army-run hospital in Bangkok, the third anniversary of a military coup, and authorities said more than 20 people were wounded.

People gather in front of the entrance of Phramongkutklao Hospital, a military-owned hospital in Bangkok that is also open to civilians, after a bomb wounded more than 20 people, The deputy commissioner of the Royal Thai Police said investigators found traces of batteries and wires at the scene.

“We can confirm at this stage that it is a bomb,” he said.

It was not immediately clear who was behind the explosion or if it was linked to the anniversary of the 2014 military coup.

But the army chief, Gen. Chalermchai Sittisart, said it appeared that the explosion and two earlier blasts in recent weeks used similar explosive materials and were likely part of an attempt to disrupt the government.

“All of this was conducted with the goal of creating disorder to the administrative work of the government and NCPO,” he said.

But he cautioned that “we shouldn’t conclude anything yet” about who was behind the attack.

The blast wounded 21 people, one of them severely, said Lt. Gen. Saroj Kiewkajee, a hospital official. Thirteen were discharged soon after the explosion.

Phramongkutklao is a military-run hospital that is also open to civilians. The blast radius from the explosion was 2 to 3 meters, police said.

Since the 2014 coup, at least six explosions have occurred in Bangkok.