Police were looking for a 26-year-old Thai woman and a foreign man in his 40s after expanding their search to a property in the city's Min Buri district. They found fertiliser, digital watches and an explosives detonator.
That came after a raid on an a decaying apartment building in the nearby Nong Chok district on Saturday, when police arrested a foreigner and seized several kinds of explosives and more than 200 passports.
The Aug. 17 attack on a Bangkok Hindu shrine killed 20 people and injured more than 100. Fourteen foreigners were among those killed in a blast the military government said was aimed at dealing a blow to an already ailing economy for which tourism has become crucial.
A sketch of the man showed him with cropped hair and a short moustache. A picture of the female suspect, Wanna Suansant, showed her wearing a hijab. She rented the room occupied by the foreign man, for whom police issued the second arrest warrant, spokesman Prawut Thavornsiri said.
Police visited the family of the woman in southern Phangna province soon after the warrant was issued and were told she was living overseas.
"They said Wanna lives in Turkey with her Turkish partner and her child. She has not contacted them for a while," deputy provincial police chief Taracha Thompat told Reuters by phone.
Police have been looking into a possible Turkish connection in their probe and last week said they were checking arrivals from that country.
Fake Turkish passports were seized and police requested a Turkish translator to assist in questioning the suspect arrested on Saturday.
The Turkish embassy in Bangkok was not immediately available for comment.
Prime Minister Prayuth Chan-ocha indicated at least one of the two suspects had been caught on closed-circuit cameras on the day of the bombing. However, he did not specify the location covered by the surveillance video.
Police have been criticised for an erratic investigation that had, until this weekend, uncovered few clues about who was
behind the blast. No group has claimed responsibility.
The bright pink budget apartment in Min Buri was raided twice at the weekend and media were allowed to observe searches on Sunday while residents, many of them Muslim, were present.
Police have not confirmed the identity or nationality of the
28-year-old man they arrested on Saturday in the busy Nong Chok, area, where many Thai Muslims and foreigners live.
He is charged with possessing illegal explosives. His identity and nationality are unknown.
Army chief General Udomdej Sitabutr said the man was definitely involved in the shrine bombing and authorities were trying to extract information from him, but withholding some to avoid jeopardising the probe.
Speculation on who was involved has focused on southern ethnic Malay rebels, opponents of the government, foreign extremists and sympathisers of Uighur Muslims, among other groups. Thailand drew international outrage last month when it forcibly repatriated more than 100 Uighurs to China.
Many of the minority Uighurs have sought passage to Turkey via Southeast Asia.
"We don't want our home to be the rest stop, or the passing point of unusual migration," Prayuth said. "This is where the network may be hiding. That's why this could also be a reason... a conflict because authorities are becoming more strict."
