Forensic tests on two suspects have failed to find a link to the site of Thailand's deadliest bomb attack, police said on Friday, dealing a blow to the investigation.

DNA examination of the two foreigners tie them to a stash of explosives found in a Bangkok apartment block, but not to evidence collected at the Hindu Erawan Shrine where 20 people were killed on Aug. 17, police said in a televised announcement.

The lack of a link complicates a high-profile case shrouded in mystery, with authorities no closer to establishing a motive for the attack carried out in one of Bangkok's busiest commercial areas.

The military has speculated the perpetrators could have been members of a human trafficking gang frustrated by a police crackdown. Thailand has rejected the possibility a militant group was involved.

Police were testing DNA samples of the second of two foreigners, to establish if he was the chief suspect - a yellow-shirted man caught on surveillance camera placing a rucksack at the shrine before the explosion.

"There's no evidence to confirm he is the yellow-shirt man," police spokesman Prawut Thawornsiri told reporters.

Prawut said police believed he was "definitely involved in the bombing".

Police seized a large amount of bomb-making material in raids on two buildings in north Bangkok, but nothing that ties the two men, whose nationalities are unknown, directly to the attack.

The bomb killed 14 foreigners, including seven from China and Hong Kong and wounded more than 100 people.

Investigators were trying to match the second detained man, who was arrested at the Thai-Cambodia border on Tuesday, with DNA left by the prime suspect in a cab, on fragments of the backpack and on a banknote given to a motorcycle taxi driver.

The man was carrying a Chinese passport which gave his name as Yusufu Mieraili, and his place of birth as the western Chinese region of Xinjiang, but it was unclear if it was authentic.

If the China link is proven it would add weight to theories by some security experts that the bombing could have been revenge by sympathisers of the mainly Turkic-speaking Uighur Muslims from Xinjiang.

In July, Thailand deported 109 Uighurs to China, where many suffer persecution. That struck a chord in Turkey, which has a large Uighur diaspora.

Police have established a firmer Turkish connection, using the language to interrogate the suspects, one of whom was arrested with fake Turkish passports. Two other suspects are believed to be in Turkey.