They also seized two pickup trucks and four mobile phones.
The suspects were identified as Samran Saenluang, 47, Pongsapak Ruangkitsuwan, 41, Somyos Laomi, 42, and Nopporn Panasirisomboon, 22, all residents of Chiang Mai.
The suspects face charges of possessing illegal drugs.
They were arrested on Thursday after Chiang Mai police received a tip-off that there would be an attempt to smuggle a large amount of methamphetamine pills into Chiang Mai’s Phrao district.
Police set up checkpoints in the district. They later stopped two suspicious pickup trucks running toward a checkpoint. They search the first pickup truck, but found nothing.
However they found 600,000 tablets of yaba in the second pickup truck.
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The suspects denied the charges, saying that they had only recently bought the vehicles from a second-hand car dealer and had no knowledge that there was a secret box containing the drugs.
But police learned from intelligence sources that a drug ring had paid the suspects 500,000 baht to smuggle the drugs over the nearby border into Thailand.
Under the plan, the drug would have been initially stashed in Chiang Mai and later transported to clients in Bangkok and its vicinity, according to police.