A Thai woman courier suspected of smuggling drugs into Japan has returned to Thailand after convincing Japanese prosecutors she was unaware that crystal methamphetamine was hidden in coffee sachets in a package she carried for a client.

Meanwhile, Thai investigators have expanded their probe into what is believed to be a Vietnamese drug trafficking network that misused her services.

Juthathip Thongnoi, 28, returned to Thailand on Saturday after Japanese authorities decided not to indict her,

She told Thai police on her arrival back that she had inspected the parcel before travelling to Japan, but had no reason to suspect it held crystal meth, or "ice", concealed inside resealed coffee packets.

Pol Lt Gen Archayon Kraithong, commissioner of the Narcotics Suppression Bureau, said on Sunday that Ms Juthathip maintained she had no knowledge of the drugs.

She stated that she had received a parcel from a woman known as "Ing Ing" and had opened the box. Inside was a range of consumer products, which she removed from the box and repacked in her luggage.

She admitted she did not thoroughly inspect every individual item, and therefore did not discover the drug concealed in the coffee sachets.

Ms Juthathip also told investigators that the 3,600 baht she received for carrying the parcel did not strike her as unusual. She regularly carried goods for Thai customers in Japan and payment varied depending on the number of items and other trip arrangements.

Pol Lt Gen Archayon said coffee packets can be resealed using handheld plastic-bag sealers that are widely available and their use difficult to detect without close inspection.

- Vietnamese network targeted -

Thai investigators said the "Ing Ing" who paid Ms Juthathip to deliver the parcel may be the same woman that a male Vietnamese suspect identified as "Bang".

Bang entered Thailand on July 8 and was later seen purchasing items believed to include coffee products at a shopping centre in Bangkok's Bang Kapi district.

Police believe the coffee in the sachets was replaced with crystal meth before the parcel was passed to Ms Juthathip for delivery to Japan.

Bang had left Thailand on July 14, the same day Ms Juthathip flew out was arrested on arrival at Fukuoka Airport.

Investigators have obtained two arrest warrants for members of the Vietnamese network on charges of participating in a transnational criminal organisation. One of the suspects has been arrested, the other remains at large.

Police are investigating those behind the operation and the intended recipients of the drug in Japan, and were working closely with Japanese authorities on the case, Pol Lt Gen Archayon said.