Police probing Thailand's deadliest bombing issued arrest warrants on Monday for two suspects after a raid on a suburban apartment block uncovered possible bomb-making materials.
Police were hunting for a 26-year-old Thai woman and a foreign man in his 40s after a weekend search on a property in the Min Buri district uncovered fertiliser, digital watches and an explosives detonator, police spokesman Prawut Thawornsiri said on TV.
That came after a raid on an apartment in the nearby Nong Chok district on Saturday, when police arrested a foreigner and seized explosives and stacks of passports.
The Aug. 17 attack on a Bangkok Hindu shrine killed 20 people and injured more than 100.
Fourteen foreigners, seven from mainland China and Hong Kong, were among those killed in a blast the junta said was intended to cripple an already flagging economy.
"We have expanded our search to various residential building after the arrest of one suspect," Prawut said.
A picture of the female suspect showed her wearing a hijab. She rented the room occupied by the foreign man, for whom police issued the second arrest warrant, Prawut said.
A sketch of the man showed him with short hair and stubble.
People hold candles for victims of last Monday's deadly blast, at the Erawan shrine in Bangkok, Thailand, August 24, 2015.
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.
Security forces raided the bright pink low-budget apartment in Min Buri twice, once on Saturday and again on Sunday.
Police allowed media to observe as they searched rooms on Sunday while residents, many of them Muslim, were present. In some rooms, possessions were spread out on the floor.
Police have not confirmed the identity or nationality of the 28-year-old man they arrested on Saturday in Nong Chok. Many Thai Muslims and foreigners live in the district dotted with colleges, factories, rice paddies and mosques.
He is charged with possessing illegal explosives.
Police have since the weekend mentioned an unspecified network they are tracking via cellphones, but have provided no details about the group.
Since the bombing, speculation had focused on groups that could have the motive and capability to carry it out.
These have included southern ethnic Malay insurgents, opponents of the military government, foreign militant groups and sympathisers of Uighur Muslims. Thailand forcibly repatriated more than 100 Uighurs to China last month, prompting international outrage.
Many of the minority Uighurs from China's far west have sought passage to Turkey via Southeast Asia.