BANGKOK - A bomb attack in Bangkok in which 20 people were killed was likely planned weeks in advance by a group of 10 people, Thai authorities said on Thursday.
But they appeared to rule out an international terrorist group and said Chinese people were not believed to have been targeted.
No one has claimed responsibility for the blast, Thailand's worst bombing, on Monday evening at a popular Hindu shrine crowded with tourists. The government has said the attack was designed to wreck an economy that relies heavily on tourism.
The elimination of foreign militant involvement would feed speculation that either Muslim separatists waging a low-intensity insurgency in southern Thailand, or domestic political activists, were involved.
National police chief Somyot Pumpanmuang said the investigation showed the attack was planned at least a month in advance by a network of at least 10 people.
"This includes those who looked out on the streets, prepared the bomb and those at the site and ... those who knew the escape route," he told reporters.
"There must have been at least 10 people involved," Somyot said, adding nothing was being ruled out.
Police are focusing their hunt on an unidentified young man seen on grainy security camera footage, apparently planting a backpack bomb at the shrine, and have appealed to Interpol for help in finding this suspect.
They have also questioned the driver of a three-wheel"tuk-tuk" taxi who drove the suspect to the shrine. Two other men seen in the CCTV footage who police had suspected were accomplices have denied involvement.
Colonel Winthai Suvaree, a spokesman for Thailand's ruling junta, said security agencies working with counterparts from allied countries had reached a preliminary conclusion that foreign militants were probably not responsible.
"The incident is unlikely to be linked to international terrorism," Winthai said.
The city-centre Erawan shrine is particularly popular with tourists from China and other East Asian countries, and 14 foreigners were among the dead, seven from mainland China and Hong Kong. China is an important ally and trade partner for Thailand and the biggest source of foreign tourists.
Winthai said Chinese tourists were not believed to have been the target.
Plea for pictures
A day after police said the main suspect, dressed in a yellow shirt and shorts, with dark hair and a wispy beard, was believed to be European or Middle Eastern, they noted he might have been a Thai man pretending to be foreign.
They later said the tuk-tuk driver had not been able to communicate with the man, except to agree a fare. The man had shown the driver a photograph of the shrine to indicate where he wanted to go.
Police declined to say where the man had hailed the tuk-tuk and appealed for anyone taking photographs at the scene before the blast to submit their pictures.
One of the two suspected accomplices, spotted at the shrine on CCTV, had come forward to say he was a Thai tour guide, and the other was a tourist from China who had flown home, police said. Police spokesman Prawut Thawornsiri declined to say if the two had been cleared, pending a check on their documents.
The blast comes at a sensitive time for Thailand, which has been riven for a decade by a sometimes-violent struggle for power between political factions in Bangkok.
A parliament picked by the junta that seized power in 2014 is due to vote on a draft constitution next month that critics say is undemocratic. The government has promised to restore democracy next year.
The government is also dealing with flagging growth and appointed a new finance minister and a deputy premier to oversee the economy.
The Erawan shrine has reopened, with visitors leaving messages, flowers and candles.