The attack occurred on Monday as worshippers crowded into the Hindu shrine in the Thai capital, shredding bodies and incinerating motorcycles. A majority of the victims were Asian tourists.
Police on Wednesday released a sketch of a young man shown on security footage walking into the shrine and placing a backpack believed to contain the bomb, which detonated a few minutes later.
A Bangkok court issued an arrest warrant for the man, describing him as a tall foreigner with white skin and accusing him "of premeditated murder, attempted murder and bomb-making".
Police said he was part of a "network", and that two other men seen on the security footage at the shrine before the blast were suspected to have helped him.
National police spokeseperson Prawut Thavornsiri said the prime suspect had been overheard speaking a foreign language other than English.
Prawut also said that the suspect looked like he may have been from the Middle East, South Asia, or Central Asia, using a Thai term to describe light-skinned Muslims from those regions.
The attack, in a popular tourist hub and close to upscale shopping malls, claimed the lives of at least 13 foreigners -- from China, Hong Kong, Britain, Indonesia, Singapore and Malaysia.
Another 68 people remained in hospital on Wednesday, 12 of whom were in critical conditions.
Prime Minister Prayut Chan-O-Cha described the bombing as the "worst ever attack" on Thailand.
After nearly 48 hours of mystery over who carried out the assault, the arrest warrant and comments by Thai police showed they suspected a heavy foreign involvement.
But with no group claiming responsibility, Bangkok residents had little choice but to guess over which foreign groups may have had the motives and capabilities to carry out the attack.
Some Thai media outlets cast suspicion on militants from China's Uighur minority, a group that faces cultural and religious repression in its homeland.
Chinese authorities have blamed Uighurs for a series of attacks inside China, most of which have been knife assaults.
Last month Thailand forcibly repatriated more than 100 Uighur refugees to China, and some Thai media speculated that Monday's attack may have been an act of revenge.
But Uighur groups are not known to have ever carried out an attack outside China.
The Erawan shrine is enormously popular among ethnic Chinese visitors from across Asia.
Various Islamic militant groups have also carried out many attacks in other parts of Southeast Asia, including bombings on Indonesia's holiday island of Bali in 2002 that killed 202 people.
But they have not previously made Thailand a prime target.
The Islamic State group, which has caused carnage in the Middle East, is not known to have carried out any attacks in Southeast Asia.
The police sketch of the suspected bomber showed him with black-rimmed glasses, a full head of dark hair and a light complexion.
Police spokesperson Prawut said his features were gleaned partly from security footage.
But he said the profile had also used information gathered from interviews with the drivers of motorcycle taxis the suspect had encountered, including the one he used to leave the shrine.
Motorcycle taxis are a common form of transport in Bangkok.
The security footage, released to the public on Tuesday, showed the suspect wearing a bright yellow T-shirt and dark shorts, walking into the shrine with a backpack.
He casually placed the backpack underneath a bench and then slowly walked away clutching a blue plastic bag while looking at what appeared to be a smartphone.
Police said he escaped on a motorcycle taxi a few minutes before the blast.
Prawut said the other two suspects were seen on the footage close to the suspect as he slid his backpack under the bench, and may have been deliberately shielding him from passers-by.
Authorities on Wednesday offered a one million baht ($28,000) reward for information that leads to the arrest of the main suspect.
Police had initially said a second explosion at a Bangkok pier on Tuesday that caused no injuries may also be linked, deepening fears for residents of their safety.
But on Wednesday they said no link could definitively be drawn, and it may have just been a copycat.
Security experts had previously said they were baffled over who was responsible.
Thailand has experienced a near-decade-long political crisis that has seen endless rounds of street violence, but nothing on Monday's scale nor targeted at foreigners.
An insurgency by Muslim rebels in the Thai south has claimed 6,400 victims -- the majority civilians -- but the rebels have not taken their conflict to Bangkok.
The shrine, a popular tourist attraction that typifies the kingdom's unusual blend of Hindu and Buddhist traditions, reopened on Wednesday morning with a ceremony led by chanting monks.
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