The deputy national police chief said on Monday that the explosion in an Army hospital in Bangkok that morning was caused by a bomb.
Most of the 10 people injured were retired Army officers who were waiting to receive medicine.
Pol General Srivara Rangsibhramanakul was speaking after investigating the explosion scene at a reception room for retired army officials at Phramongkutklao Hospital in Rajthevi district.
The explosion took place on the third anniversary of the coup led by then Army commander-in-chief General Prayut Chan-o-cha, now the prime minister.
Initial reports said most of the injured were showered with broken window glass.
At first, it is reported that the explosion may have been caused by gas leak or a faulty air-conditioner compressor.
Srivara said at this stage, the explosive used in the attack was unknown but an integrated circuit timer, pieces of wires and a battery were found at the scene. Srivara said he would hold an urgent meeting.
He said he would wait for bomb squads to collect the evidence before deciding whether the attack was linked to the explosion in front of old buildings at the Government Lottery Office on April 5 and the explosion in front of the National Theatre on May 15.
Srivara condemned the attack at the hospital, saying even the southern insurgents have never attacked a hospital.
The injured were treated in the emergency unit of the hospital.