An unidentified bomber attached a green vase with artificial flowers to a wall in the reception room of Phramongkutklao Hospital before a homemade bomb exploded there on Monday, a police source said.
Police investigators of the Metropolitan Police Bureau have obtained a picture of a vase that was taken just 10 minutes before the explosion, the source added.
The picture shows the green vase attached to the wall while three people wait on seats.The reception room is a VIP room where military officials and retired officers wait to receive medicine.
The bomb exploded at 10.40am, injuring 25 people.
The police source said the attacker apparently passed among patients to put up the vase. Staff of the hospital said they did not see the vase earlier.
Police are still checking CCTV footage to try and isolate the attacker.