BANGKOK—A bomb exploded at a hospital in Bangkok on Monday, wounding more than 20 people on the third anniversary of a military coup.

Investigators found traces of batteries and wires at the scene of the blast at Phramongkutklao Hospital, according to Pol. Gen. Srivara Ransibrahmanakul, the deputy commissioner of the Royal Thai Police.

The blast radius was 2 to 3 meters wide, he said.

It wasn’t immediately clear who was behind the blast or if it was connected to the anniversary of the 2014 military co