Hunt for Bangkok bomber - what we know so far

BANGKOK - Thai police are searching for a man they believe left a backpack bomb at a Bangkok shrine that exploded amid a crowd of worshippers, killing 20 people and wounding more than 100 others.

No one has claimed responsibility for Monday's attack, which occurred at the Erawan shrine in one of the Thai capital's popular tourism and upscale shopping districts.

Here is what we know about the investigation:

What do we know about the suspected bomber?

The first glimpse came a day after the blast when police disclosed grainy security footage of a man casually walking into the shrine with a backpack on.

He sits on a bench close to some iron railings that surround the shrine, puts the rucksack under a bench and slowly walks away while apparently looking at his smartphone.

Police said he left on a motorcycle taxi, a common form of transport in Thailand.

Three minutes later a bomb explodes in that exact spot.

On Wednesday the authorities released a detailed digital sketch of the suspect gleaned from higher quality CCTV footage which has not been released.

The portrait shows a young man with black-rimmed glasses, a full head of dark hair and a light complexion.

A court Wednesday issued an arrest warrant for an unnamed foreign man.

Police have not said how they reached the conclusion that the main suspect is a foreigner.

Is the bomber acting alone?

Police are increasingly confident the man seen on Monday night must have had accomplices, both to build the devices and plan the attack.

"It's a network," national police chief General Somyot Poompanmoung told reporters on Wednesday. "We believe there must be people helping him, Thai people."

"There must be a network," added Thai junta chief Prayut Chan-O-Cha Wednesday. "He can't just buy the bomb in a market and then place it."

Spokesman Prawut said police believed "at least three people" were involved in the plot.

Who has suspicion fallen on?

Many groups have been discussed as potential culprits, but security experts say there are strong reasons to discount each and are baffled over who is responsible. Some of the groups are:

Political rivals:

Bangkok has endured a decade of deadly political violence amid a power struggle between the military, backed by the middle class and elite, and a rural and urban poor loyal to ousted populist former premiers Yingluck and Thaksin Shinawatra.

The power struggle has seen repeated rounds of deadly street protests.

But experts say neither side had much to gain by launching an attack of Monday's scale, risking opprobrium from the Thai public.

Local Muslim insurgents:

Insurgents are fighting for greater autonomy in the country's three Muslim-majority states bordering Malaysia annexed by Thailand a century ago.

More than 6,400 people -- mostly civilians -- have died in the last decade of conflict there.

But there are no indications Muslim rebels have suddenly taken their localised fight beyond southern Thailand.

Uighurs:

Some Thai media outlets have pointed a finger of suspicion at militants from China's Uighur minority. 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.

But Uighur groups are not known to have ever carried out an attack outside China while their domestic assaults have shown none of the technical sophistication of the Bangkok bombing.

Southeast Asian militants:

Various Islamic militant groups have carried out many attacks in other parts of Southeast Asia, including 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:

Militants from the so-called Islamic State have made it clear they wish to export or inspire militant attacks outside of Iraq and Syria.

"They've done a good job of recruiting Malaysians and Indonesia, so it's possible," Zachary Abuza, an expert on Southeast Asian militants groups told AFP.

But Abuza said IS networks usually claim credit swiftly for their attacks while Thailand's Muslim militant groups have been resistant to overtures from global jihadis.

Someone with an anti-China grudge:

The attack on the shrine itself, in a nation where spirituality and respect for faith is assumed, has baffled experts.

But the shrine's popularity with ethnic Chinese visitors -- and the fact that a second attack on Tuesday took place near Bangkok's Chinatown -- has led to speculation the blast may have specifically targeted that demographic.

What about the second attack?

On Tuesday, shortly after 1:00 pm, one or more people threw an explosive device from a bridge into a canal near the busy Saphan Taksin Skytrain station. No one was harmed.

Police have yet to decide whether the two incidents are linked.

Colonel Kamthorn Ouicharoen, head of Bangkok's bomb squad, told reporters both devices were "exactly the same".

But police chief Somyot Poompanmoung has said that attack could also be an unrelated or "copycat" act.