Security officials are checking guesthouses, apartments and places popular among foreigners in their bid to catch the suspected bomber behind the deadly Erawan Shrine blast.
The blast killed 20 people and injured more than 100 last Monday.
"There are more than 10,000 places in Bangkok that we have to inspect and search," Metropolitan Police chief Lt-General Srivara Ransibrahmanakul said. "We may also need to repeat searches at some places." Srivara said investigators had interviewed more than 10 witnesses and planned to interview all survivors of the blast.
Asked about Japanese media reports that the suspected bomber was a Spanish man, Srivara said he had not yet seen those reports.
No arrest has been made yet in connection with the blast, described as the worst ever attack on Thai soil.
National police chief Somyot Poompanmuang said yesterday that all relevant officials had been working hard to solve this case. "We have had a clear focus. It's just that we can't disclose details at this time," he said.
He said the delay in the investigation was related to the lack of modern equipment, not the ability of officials.
Somyot spoke after he oversaw the start of the "Searching the city to crack down on criminals' dens " operation yesterday morning. Participating in the Bangkok-based operation were police, soldiers and Bangkok Metropolitan Administration police.
The operation was aimed at restoring local residents' and tourists' confidence in the wake of the blasts, he said. Asked if police knew who was behind the shrine attack, Somyot said, "We have had some information. Some of those involved are still in Thailand."
But he refused to say if the suspects were Thais or foreigners.
Security footage showed a man in a yellow-shirt, who did not look Thai, walking into the Erawan Shrine with a backpack just before 7pm on August 17 and leaving without it a few minutes later. The blast occurred shortly after he walked out. He was last seen getting onto a motorcycle taxi.
An informed source revealed yesterday that investigators had already summoned 15 taxi drivers for questioning to determine if any had given a ride to the suspected bomber on that day. "He's also seen getting into a taxi but grainy footage makes it hard to identify the licence plate number."
In regard to a second bomb that exploded harmlessly near Sathorn Pier the following day, August 18, spokesman Pol Lt General Prawut Thavornsiri said police were investigating whether a man in a blue shirt had carried the bomb to that spot.
Bomber 'was an Asian'
The man in the blue-shirt was caught on CCTV and that video clip has also circulated widely, but not officially. "Our preliminary check suggests this man was an Asian," Prawut said. "He did not use the same route as the yellow-shirt man [responsible for the blast at the Erawan Shrine]".
Meanwhile, Indonesian police are currently looking into whether the East Indonesian Mujahidin (MIT) terrorist group had links to the Erawan Shrine bombing, the Jakarta Post has reported. The Post said Kompass online quoted Coordinating Political, Legal and Security Affairs Minister Luhut Binsar Pandjaitan as saying that MIT, a terrorist group in Poso, central Sulawesi, was being targeted. "We are currently pursuing [MIT]. I have received reports about their movements," said Luhut, with his reports having come from the National Police chief General Badrodin Haiti, according to the Post.
Up to 140 Mobile Brigade (Brimob) officers from Kelapa Dua in Depok had been flown to Poso to assist with the operation. The terror group is said to have 30 to 40 members.
"If we don't act, they can mobilise more power from various places, including weapons," Badrodin was quoted as saying.
There has been some speculation that the Islamic State (IS) was behind the Bangkok bomb blast, and in Indonesia, the IS movement is allegedly supported by the MIT, which is led by Santoso.
In Thailand, a National Council for Peace and Order spokesman said two rumour-mongers had been arrested for allegedly causing public confusion, one in Bangkok and the other in Ayutthaya province.