A Lebanese man suspected of planning a possible bomb attack appeared in a Thai court on Tuesday a day after leading police to a warehouse stocked with bomb-making materials and being charged under weapons laws.
Authorities, however, said the explosive ingredients were not intended for use in Thailand and were to be shipped out of the country, hoping to allay public fears following emergency warnings issued by the United States, Israel and nine other countries against possible attacks in Bangkok.
Police would seek a court order to further detain the man, Atris Hussein, a holder of a Swedish passport and linked to the Hezbollah, to carry out further interrogations over his role in an alleged plot that has been shrouded in mystery.
"We're seeking to detain him for four more days, which can be extended for up to 48 days," metropolitan police spokesman Piya Uthayo said on Tuesday.
Hussein was arrested at Bangkok's Suvarnabhumi Airport late on Thursday and led police to a Bangkok suburb where 4,380 kg of urea and 37 litres of liquid ammonium nitrate, a chemical compound, were stored.
Urea and the chemical ammonium nitrate have been used in a number of deadly attacks, such as the Oklahoma City bombing in 1995, which killed 168 people, and a car bomb that exploded outside the Norwegian prime minister's office in Oslo in July last year, which killed eight.
But the amounts stored in Bangkok were exceptionally large, roughly seven times bigger than the urea-nitrate and hydrogen mixture used in the 1993 bombing of the World Trade Centre in New York and twice the amount used in the Oklahoma bombing.
Ammonium nitrate and urea are also widely used for home-made bombs to target soldiers and government buildings in Thailand's three Muslim southernmost provinces, where an insurgency has killed nearly 5,000 people since 2004.
"This is obviously a serious wake-up call for the Thai authorities and it is surprising given supposedly tighter restrictions on home-made explosives of this type, driven by their extensive use in the insurgency in the south," said Anthony Davis, a Bangkok-based security analyst with IHS Jane's.
"They will be focused on establishing exactly where these materials were sourced, by whom and over what period of time."
Piya said Hussein had been charged under the country's Arms Control Act, which requires defence ministry licences for possession of large quantities of substances that can be used to make bombs.
Breaches of that law are punishable by up to five years in prison if found guilty.
Thai officials say another suspect has fled the country and that Hussein told them Thailand was not the target of any plot.
The United States and Israel, and subsequently nine other countries, issued terrorism warnings on Friday to their citizens about possible attacks in parts of the kingdom frequented by Westerners, statements that have irked Thailand, which is concerned about negative impact on its tourism industry.
Tourism employs 15 per cent of Thailand's workforce and is worth roughly 6 per cent of gross domestic product.
Prime Minister Yingluck Shinawatra has called for calm and asked for the warnings to be withdrawn on the grounds that there was yet no evidence Thailand would be targeted.
The United States, a longtime ally of Thailand, has refused and said its warning was credible.
Hezbollah was suspected of involvement in plot to attack the Israeli embassy in Bangkok in 1994, when a one-tonne truck bomb was discovered but not detonated.
Both chemicals found on Monday are sold for legitimate purposes -- farming, gardening, mining and for golf courses -- often in very large quantities and the US. Department of Homeland Security in August proposed restrictions on their sale.
Security has been stepped up in tourist sites in Bangkok since the warnings were issued, with a bigger police presence outside Western embassies, airports and Khao San Road, a bustling backpacker strip crammed with bars, guesthouses, shops and restaurants.