Investigators of the bombing at Phramongkhutklao hospital have now strongly believed political motive was behind the attack while ruling out southern insurgency link.
They said politics was the most probable cause as evidence collected so far and many circumstances were coincidental.
The explosion happened in front of the Wongsuwan room inside the Chalermphrakiat building of the hospital.
Wongsuwan is the family name of deputy prime minister Gen Prawit Wongsuwan. He was reported to be sick and treated at the hospital .
The explosion happened on May 22, which is the day when the military junta seized power from the Yingluck Government on May 22, 2014.
The hospital is also the army hospital.
Earlier Prime Minister Gen Prayut Chan-o-cha also voiced his suspecions of all coincidental incidents but didn't point finger at any particular group.
Although investigators did not pinpoint any certain group, but they all said the bombing was carried out by well-organised group with link to overseas.
Royal Thai Police deputy commissioner Pol Gen Srivara Rangsibhramanakul said the bombing was more closely resembled the five cases of bombings during the 2007 political disturbances.
There was no evidence to indicate that the hospital bombing was in any way connected to the bombings in the southern provinces, he said.
The connection to the 2007 political disturbances is backed up by the similarity of the bomb fragments and explosive material that has been collected by forensics experts.
Meanwhile police revealed that 3 days prior to the hospital bombing, a warning was sent to the Thailand's National Cancer Institute which located near Phramongkutlklao hospital by an unknown individual.
Police yesterday revealed the pictures of the green porcelain vase that was used to conceal the bomb.
The pictures were taken by a visitor to the hospital before it exploded.
Before it went off, it held a bunch of orange flowers and was attached to a wall at the scene.
A closed circuit TV footage initially showed that there was one individual very close to the vase prior to the bombing.
Commenting on this, Pol Gen Srivara stated that investigators have now discovered that the person in the clip was in fact one of the victims whom was also injured when the bomb went off.
Investigators have questioned the person as well as other victims and will be adding their accounts to the case file.
Examination of closed circuit TV footages however has failed to produce leads on the perpetrators but police investigators stated that they will be employing other investigative means to identify the bomber.
Source: Thai Public Broadcasting Service (Thai PBS)