Earlier in the investigation, police said they believed at least three people, including a foreign man, were involved in Monday's attack on the famous Erawan shrine.
“It is a big network. There was preparation using many people,” police chief Somyot Poompanmoung told reporters on Thursday.
“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 said. “There must have been at least 10 people involved.”
Thailand has asked Interpol for help in the investigation, sending the international police organization an image of one of the suspected bombers.
“We sent a request for assistance,” said deputy national police spokesman Kissana Phathancharoen.
The military government was initially reluctant in asking for outside help. Hours earlier, Thai Prime Minister Prayuth Chan-o-cha reportedly rejected Britain’s offer of assistance in the investigation.
“Do you want foreign countries to intervene in every issue? No country will do this. It is a breach of sovereignty,” Prayuth was quoted as saying in English-language newspaper Bangkok Post.
British national Vivian Chan, 19, was among those killed in the attack, the Post reported.
Thailand's military government spokesman, Col. Winthai Suvaree said on Thursday that the bombing was “unlikely to be linked to international terrorism.”
“The security agencies have collaborated with intelligence agencies from a lot of countries and have come to the same preliminary conclusion that the incident is unlikely to be linked to international terrorism,” he said. Although the Erawan shrine is popular with Chinese tourists, Winthai said that they were not the “direct target” of the attack.
There has been no claim of responsibility and police have not determined a motive for the worst bomb blast in Thailand.
Police suspect the young man whose image was caught in grainy footage leaving a backpack at the crowded shrine shortly before the explosion is foreign but Kissana said Thai police were not focused on any particular country or region with their appeal to Interpol.
“We basically sent in the modus operandi [of the suspect] and also the appearance of the suspect we're looking for,” Kissana added.
The government says the attack was aimed at undermining Thailand's heavily tourism-dependent economy. About half the victims were foreigners.
In the aftermath of the bombing, some foreign travel agencies cancelled group tours and the baht sank to a six-year low on Wednesday. The country's stock benchmark fell to its lowest since the start of the year, led by tourism-related shares.
Tourism had just begun to rebound after a military junta seized power in a 2014 coup. The arrival numbers of the all-important Chinese market doubled for the first half of this year compared to the same period last year. The Erawan shrine at a main city-center intersection is popular in particular with tourists from China and other East Asian countries.
Police have issued an arrest warrant for an “unidentified foreign man” in a yellow shirt seen in the video footage and have said investigators believe two other men seen on the footage were accomplices.
Checks at airports and other exit points found that no one matching the description of the main suspect had left the country since the attack, Kissana said.
On Tuesday, a small explosive device was thrown at a busy pier on Bangkok's main river but no one was hurt. Police have not ruled out a link between the two blasts.
Before Winthai downplayed an international link, police said they were considering the possibility that ethnic Uighurs were behind the bombing.
Hundreds, possibly thousands, of members of the Turkic-speaking and largely Muslim minority have fled unrest in China's western Xinjiang region, where hundreds of people have been killed, prompting a crackdown by Chinese authorities. Many Uighurs have traveled through Southeast Asia to Turkey.
Thailand forcibly returned 109 Uighurs to China last month.
But Police Lt. Gen. Prawut Thavornsiri described the chief suspect as “Caucasian, Arab or mixed race,” amid what the Bangkok Post reported were attempts to “play down” potential Uighur involvement in the blast.
The Erawan shrine, which is popular with Buddhists in Thailand, has since reopened and visitors have left messages of condolence, flowers and candles at the site.
Al Jazeera and wire services