More than 21,000 "yaba" tablets, with a street value that exceeds $540,000, have been seized in the largest single haul of the drug in 14 years.
The methamphetamine tablets were found in a car on Tuesday at Woodlands Checkpoint.
Three men and a woman have been arrested, said the Central Narcotics Bureau (CNB) and the Immigration and Checkpoints Authority (ICA) in a joint statement Wednesday.
On Tuesday, at about 7.45pm, a Thai-registered vehicle driven by a 51-year-old Thai national arrived at the checkpoint. He was arrested immediately when four black packages, with 21,910 "yaba" tablets, were found hidden behind the car's glove compartment during an ICA inspection.
If convicted, he may face the death penalty.
Following his arrest, the CNB carried out two raids that led to the arrest of two Malaysian men and a Singaporean women in Jalan Sultan Wednesday.
The trio, aged between 35 and 41, are suspected of being connected to the Thai national and the drug consignment. Investigations on possible drug trafficking activities by the trio are ongoing.
Said CNB director Ng Ser Song: "Together with our Home Team partners, CNB will maintain our relentless enforcement at the checkpoints to prevent individuals or drug syndicates from smuggling in illicit drugs."
Methamphetamine in its crystal form is also known as "Ice".
It has been named as the top illicit drug in South-east Asia by the United Nations Office on Drugs and Crime.
Some 22.49kg of Ice were seized by CNB in the first six months of the year, more than double the haul in the same period last year.
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