BANGKOK (AFP) - Thai police searching for a suspected network behind a deadly bomb attack on a Bangkok shrine last week made a potential breakthrough on Saturday, arresting and charging a foreign man.
No one has claimed responsibility for the unprecedented bombing - which killed 20 people, mostly ethnic Chinese tourists - at a shrine in a bustling downtown district.
Many groups have been discussed as potential culprits, but security experts say there are strong reasons to discount each and are baffled over who might have carried out the attack or why.
Until now an under pressure police force had seemingly struggled to find a clear motive or name a suspect.
Here is what we know about the investigation so far:
What happened in the Bangkok attack?
Shortly before 7pm on Aug 17, a powerful pipe bomb exploded inside the Erawan shrine.
The investigation soon focused on an unnamed man in a yellow T-shirt seen placing a backpack in the same spot where the bomb detonated minutes later.
The following afternoon an explosion occurred in a canal near Saphan Taksin, a busy riverside transport hub.
The prime suspect in that attack was a man in a blue t-shirt seen kicking a bag into the canal the day before.
No injuries were caused in that attack but police believe the two incidents are linked.
How did the police find their man?
After days of no arrests or named suspects, investigators swooped on Saturday morning on a four-storey apartment block in Nong Chok, an eastern suburb of Bangkok with a significant Muslim population.
Police say a foreign man was discovered there with bomb-making equipment, including pipes and ball bearings similar in size to those used in the shrine and canal attack, as well as dozens of passports.
They released photos of a slim, dishevelled man detained at the flat, sporting a light beard.
He was later taken away with a hood over his head as locals looked on.
Investigators have remained tight-lipped on what led them there.
Who is he?
That is what Thai police are now trying to ascertain.
Police say he is foreign but have yet to confirm the man's nationality.
During a televised broadcast announcing the arrest they showed stacks of Turkish passports found in the suspect's flat and a passport photo page of a 28-year-old Turkish national sporting similar looks to the arrested man.
However, in a city renowned for being a passport forging hub, questions were quickly raised over errors on the page.
They included an unusual spelling for Istanbul, the word birth spelt "brth" and "Date of expiry" appearing twice.
A senior military officer told AFP that the man was believed to be Turkish but that authorities were working to verify his nationality with the local embassy.
Who might have carried out the attack?
Uighurs or sympathisers: The fact that a shrine beloved by Chinese devotees was targeted has led some to speculate whether it might be revenge for Beijing's treatment of the Uighurs.
Rights groups say the Turkic ethnic minority has long suffered repression in western China.
Chinese authorities have blamed Uighurs for a series of attacks inside China, most of which have been knife assaults.
Some Uighurs have fled to Thailand presenting themselves as Turkish.
Last month, Thailand forcibly repatriated more than 100 Uighur refugees to China, insisting they return to their homeland, sparking the storming of a Thai consulate in Turkey where support for the Uighurs runs high.
Some analysts have suggested pan-Turkic nationalists or those wanting to avenge the Uighurs may have carried out the attack.
Uighur groups themselves 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.
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 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 on Monday's scale, risking opprobrium from the Thai public, as well as bad karma for hitting such a beloved shrine.
Local Muslim insurgents: Insurgents are fighting for greater autonomy in the country's three Muslim-majority states bordering Malaysia and annexed by Thailand a century ago.
More than 6,400 people - mostly civilians - have died in the last decade in the conflict.
But there are no indications Muslim rebels have suddenly taken their localised fight beyond southern Thailand.
International Islamic militants: Various Islamic militant groups have carried out many attacks in other parts of South-east Asia, including the 2002 Bali attacks that killed more than 200.
But they have not previously made Thailand a prime target.
The so-called Islamic State in Iraq and Syria, or ISIS, has also had some success in recruiting South-east Asian militants.
But analysts say Islamist militants crave publicity and would always claim their attacks.