The suspected mastermind of last month's Erawan shine bombing left for China from Bangladesh 12 days ago, and efforts in Malaysia to track the man suspected of planting the explosives have been unsuccessful, police said Friday.
No group has claimed responsibility for the Aug 17 attack that killed 20 people, including 14 foreign tourists, among them seven from Hong Kong and mainland China.
Reuters reported that the man Thai police believe organised the plot and fled on the eve of the blast spent two weeks in Bangladesh before taking a flight from Dhaka to Beijing on Aug 30, Bangladesh police said.
"We checked his name, arrival date and passport number with the immigration department ... we found that it was correct and he left from the same airport (to Beijing)," Shahidul Hoque, inspector general of Bangladesh police, told Reuters.
Mr Hoque said the name on the passport was Abudustar Abdulrahman, adding his men were looking for the suspect's hotel.
In Beijing, Chinese Foreign Ministry spokesman Hong Lei said he was not aware if the suspect had flown to China from Bangladesh. "This case is still under investigation," he told a daily briefing.
National police chief Somyot Pumpunmuang downplayed the Reuters report, saying it may only be a rumour, one of many.
Thai police have not yet asked for cooperation from Bangladeshi authorities as investigators had to verify the information. However, all immigration checkpoints nationwide have been alert to look for the suspects.
Pol Lt Gen Prawut Thavornsiri, the police spokesperson, said authorities had verified with the Thai embassy in Dhaka that Ishan had arrived in Bangladesh and the ambassador there already asked to discuss the issue with that country's national police chief on Friday or Saturday.
He admitted police had no information about him supposedly leaving for China, again calling it a rumour.
Pol Lt Gen Prawut said a formal arrest warrant still had not been issued for Mr Abdulrahman, as some suspects said he stayed with them, but did not implicate him in the actual bombing. Therefore, at this stage, he was a person of interest.
Thai police have ruled out international militants as responsible for the attack but have not said why nor what they believe the motive was.
Police said on Friday the investigation had expanded to Malaysia.
Suchart Teerasawat, a police inspector general, told Reuters he was in Malaysia last week following a lead that a yellow-shirted man caught on a security camera leaving the bomb at the Erawan shrine may have crossed the southern border.
"We have coordinated our efforts with the Malaysian police," he said, while adding he had no concrete information.
The investigation gained momentum after information was given by one of two detained suspects, who was arrested with a Chinese passport bearing the name Yusufu Mieraili.
He admitted to giving a bag containing the device to the bomber and said another man, Abu Dustar Abdulrahman who is also known as "Ishan", had given orders to the plotters. Police had tracked that man to Bangladesh.
Thai visas were issued in Kuala Lumpur to both men, according to a diplomatic source, using Chinese passports with names Abu Dustar Abdulrahman and Yusufu Mieraili, in October 2014 and February 2015 respectively.
The use of Chinese passports, at least one of which stated Xinjiang as a birthplace, adds fuel to speculation the bombing may have been a revenge attack by sympathisers of Turkic-speaking Uighur Muslims.
Thailand was widely condemned in July for forcibly repatriating 109 Uighurs to China, where they say they are persecuted. Many Uighurs from western China's Xinjiang region travel though Southeast Asia to get to Turkey. China denies persecuting Uighurs.