The suspect, described as a foreigner, was charged with illegal possession of weapons, but was not the man seen on CCTV footage leaving a bag at the Erawan Shrine before the explosion.
The unnamed 28-year-old was arrested on Saturday after police raided an apartment he was in where they found bomb-making equipment and fake passports.
It was the first possible breakthrough in the investigation into the August 17 blast which killed 20 people, more than half of whom were foreigners, and injured more than 120 people.
National police spokesman Prawuth Thavornsiri said his nationality and motive were still not known. He said police were exploring the theory that he was part of a network that provided fake passports to migrants.
He said police were working with “a number of embassies” and interpreters to try to establish the man’s nationality, adding that he did not speak Thai but spoke some English - and the interrogation was going slowly.
“He is not cooperating much. From our preliminary investigation, we think he isn’t telling us the truth,” Prawuth said, declining to elaborate. “He told us how he entered Thailand but we don’t believe everything he says.”
Authorities have dodged questions about whether the suspect is believed to be Turkish, saying that he was travelling on a fake passport. Images circulated online after his arrest of a fake Turkish passport with the apparent suspect’s picture.
“We don’t know if he is Turkish or not,” he said. “The passport you have seen is fake.”
No one has claimed responsibility for the blast, sparking several theories into who might be behind it. Possible suspects include parties seeking to avenge Thailand’s forced repatriation of ethnic Uighurs to China. Uighurs are related to Turks, and Turkey is home to a large Uighur community.
Other theories included Muslim separatists from southern Thailand, opponents of Thailand’s military government and feuding factions within the security services.
Until Saturday’s arrest, police had focused on a prime suspect who was seen in a security camera video leaving a backpack at a bench near the open-air shrine and then walking away.
A separate camera showed the man, wearing a yellow T-shirt, on the back of a motorcycle taxi leaving the site.
Police have been criticised for releasing conflicting statements and rapidly hosing down the crime scene at the shrine before all forensic evidence was recovered. Many accused authorities of rushing to clean up the bomb scene to reassure the public that the city was back to normal.