Police have created a sketch of a person suspected of being involved in the Phramongkutklao Hospital blast as pressure mounts on security authorities said to have been tipped off about possible attacks.
Police have not confirmed when the sketch will be released.
Deputy national police chief Srivara Ransibrahmanakul said Wednesday the blast victims were interviewed to help an artist draw a sketch of a possible suspect in the incident.
Together with other pieces of evidence, police will seek a court warrant for the arrest of the person in the sketch if there is credible evidence that the individual is involved in the blast, he said.
Pol Gen Srivara has also cleared of suspicion three men who were seen close to a vase containing the explosive device, saying they were also victims and sustained injuries.
No suspect has yet been detained three days after the explosion which took place in the dispensary's waiting room, known as the Wongsuwon room, and left 25 people injured.
The deputy police chief said the case is not easy to crack, but insisted the investigation will not reach a dead end, adding footage from surveillance security cameras at the hospital will be useful for the probe.
According to an initial investigation, the explosive device stashed in a polyvinyl chloride (PVC) pipe was placed inside a flower pot. It had an integrated circuit timer set for two to four hours.
On Wednesday, Pol Gen Srivara denied any knowledge about the detention of a man by the army in connection with the bomb attack, saying there has been no word from the military.
Asked about a letter warning regarding a series of blasts targeting hospitals, he said the letters were put in post boxes in three districts of Sam Sen, Chatuchak and Pathumwan in Bangkok.
The letters, which have different writing styles, are being examined by forensic experts who are also trying to collect DNA samples from them, he said.
He said police investigators have not ruled out any possible motives, which include politics as well the southern insurgency.
Pol Gen Srivara said all recent bomb attack cases are being reviewed to find if any of them are connected.
However, he insisted the bomb blasts have nothing to do with any foreign groups and urged the media to take extreme caution in reporting the attacks as it is a sensitive matter.
The hospital bomb blast, the third explosion in Bangkok in two months, came as the National Council for Peace and Order (NCPO) marked three years in office on Monday.
A week earlier on May 15 a bomb went off in front of the National Theatre near Sanam Luang. On April 5 an explosion took place outside the old Government Lottery Office on Ratchadamnoen Klang Avenue.
NCPO spokesman Winthai Suvaree expressed confidence that those responsible for the bomb blasts would be brought to justice.
He said the NCPO's peace-keeping corps has stepped up a security plan for public places and important venues in the wake of the hospital explosion and will make sure security cameras function properly.
Security has reportedly been beefed up at Government House and around its premises including the Klong Phadung Krung Kasem market.
Deputy Prime Minister Udomdej Sitabutr said the warning letters are still being investigated to help determine the motive behind the hospital attack.
He said authorities concerned are working at full speed to step up security for the public and bring the culprits to justice.
Prime Minister Prayut Chan-o-cha, who visited Hat Yai, Songkhla province on Wednesday, said he has some information about the Phramongkutklao Hospital blast and is following up on it.
Condemning the attack, he called on the public to denounce it too and act as the eyes and ears of the authorities to help maintain peace and order.
The CCTV cameras were mostly not working on Monday when one or more bombers planted an explosive at Phramongkutklao Hospital, so the army has got busy installing cameras that function properly. (Photo by Pattarapong Chatpattarasill)
In another development, 1st Army commander Lt Gen Apirat Kongsompong said he had information that up to four ill-intentioned groups are behind the explosion at the army-owned hospital.
"I am waiting for the order from the deputy prime minister [Prawit Wongsuwon]. I am ready to take action against these groups immediately and mercilessly as soon as he gives me the orders. We've got their names," said Lt Gen Apirat.
He said he also had information that the explosion was not carried out by foreigners, but was the work of Thai citizens.
He said the explosion undermined the economy and the confidence of investors in the country.
Meanwhile, a bomb scare was reported at Phramongkutklao Hospital Wednesday when a suspicious-looking package was reported to authorities.
An Explosive Ordnance Disposal team was sent to examine the package, which was found to only contain some clothes and other belongings. It was said to belong to a patient.
The hospital continued to function as normal, albeit amid heightened security.