The blast near Sanam Luang which injured two women on Monday night bears the signature of Bangkok's 2007 bombings, deputy national police chief Srivara Ransibrahmanakul says.
He said an initial analysis of evidence has shown the blast outside the National Theatre on Monday bears similarities to bomb incidents in Bangkok in 2007.
They included a blast at a public telephone booth in front of Major Cineplex Ratchayothin on April 9, 2007, and the explosion near the fence of the Royal Thai Army Headquarters on Ratchadamnoen Nok Road on Sept 30, 2007. Two bomb disposal officers were injured in this incident.
The 2007 bombings were believed to be politically motivated, Pol Gen Srivara said.
But he declined to say if the Monday blast was also linked to politics.
"Some evidence gathered from the scene bears the signature of the 2007 bombings in Bangkok," he said.
The deputy police chief said he had instructed Pol Lt Gen Sanit Mahathavorn, chief of the Metropolitan Police Bureau, to step up efforts to investigate the matter.
Police on Wednesday admitted the Monday blast was caused by a low-impact bomb on a timer, saying their initial claim that explosive materials were not used was a tactic to keep the investigation under wraps.
Pol Gen Srivara said a thorough examination of shrapnel and other materials at the crime scene has confirmed the blast resulted from a bomb which was big enough to stir up unrest but not likely to cause any casualties.
The bomb went off outside the National Theatre, which is not far from Sanam Luang, causing minor injuries to two passersby.
The timing of the blast led officers to question whether it was part of a larger effort to try to discredit the military-sponsored government as it came ahead of the upcoming third anniversary of the May 22, 2014 coup.
Regarding the low-impact pipe bomb that exploded in front of the old Government Lottery Office and wounded two women on Ratchadamnoen Klang Avenue on April 5, Pol Gen Srivara said Thursday that investigators had made progress in gathering evidence.
Pol Lt Gen Sanit said police are working around the clock to solve the case, and he has instructed all nine divisions under the Metropolitan Police Bureau to ensure safety in city areas under their jurisdiction.
He said evidence collected at the blast scene had been sent for forensic examination and results are expected shortly.
Also Thursday, Crime Suppression Division (CSD) police took four suspects allegedly linked to foiled bomb attacks in Bangkok and surrounding provinces last year to the Criminal Court where the CSD requested their detention for 12 days.
Police said the investigation is not yet complete.
The court approved the police's detention request.
Police opposed the suspects' bail requests, saying they posed a flight risk.
The four, among 17 suspects facing arrest warrants for an alleged 2016 bomb-campaign plot, were identified as Amri Ha, 19, Nuraman Abu, 21, Muftadin Salae, 19, and Tuanhafit Duemungkapa, 23.