SLOVAK customs officers seized around 20,000 cartons of smuggled cigarettes in Hnojné village near the Slovak-Ukrainian border on March 9.
The driver of the truck containing the contraband told customs officers that he was transporting polystyrene, news wire TASR wrote.
According to customs authorities, the truck probably crossed the Slovak-Ukrainian border on March 8 at the Vyšné Nemecké border crossing.
"We are investigating whether customs officers at Vyšné Nemecké neglected their duties," Customs Directorate spokeswoman Silvia Balázsiková said.
Tax and customs evasion represents an annual state loss of Sk10.6 million (€260,000), she added.
Slovak customs authorities assume that smugglers have been trying to stock up before Slovakia's entry to the European Union on May 1, as the country's eastern border will be better protected and monitored more closely.
Since the beginning of this year, customs officers have seized more smuggled cigarettes than in all of 2003.
