The Excise Department is investigating an allegation that one of its high-level officials tried to persuade police to let go of a truck carrying 15,000 litres of smuggled oil.
Kriangkrai Pattanaporn, deputy director-general of the department, said on Thursday that a probe committee was formed on Wednesday to look into the arrest of a 47-year-old man who drove the truck in Muang district of Prachuap Khiri Khan province.
He said an initial investigation found that the truck was transporting 15,000 litres of smuggled oil, and the truck driver was indentified only as Sombat.
Mr Kriangkrai responded to reports of anti-corruption police and excise officials having intercepted the truck on Phetkasem Highway in tambon Koh Lak, Prachuap Khiri Khan, as it was en route to the Central Plains on Saturday evening.
Following the arrest, a high-level official of the Excise Department allegedly contacted the police at scene in an attempt to secure the release of the truck and its driver. However, the police rejected the request.
Mr Kriangkrai said that his department and police of the Anti-Corruption Division are jointly conducting the investigation.
"If evidence implicates a high-level executive, the department will verify it and take disciplinary action without any leniency," the deputy director-general said.
On Thursday, crime activist Atchariya Ruangratanapong arrived at the Anti-Corruption Division, urging the agency to expand the investigation into the case.
Mr Atchariya alleged that a deputy director-general was involved in an oil-smuggling network and that the seized truck was transporting smuggled diesel from the southern border province of Songkhla to Pathum Thani province in the Central Plains.
Excise officials in Prachuap Khiri Khan were instructed to release the truck and the driver, but they were unable to do so because the highway police had already taken custody of the vehicle, according to Mr Atchariya said.
After the arrest, highway police requested that local excise officials only inspect the oil on the truck, said the activist.
He added that oil smuggling from the South occurs almost every night and raised questions regarding the whereabouts of the impounded truck.