BANGKOK — Top government officials on Tuesday kept a tight lip when questioned about recent allegations that an assistant cabinet member spent four years in an Australian jail for drug trafficking.

Prime Minister Prayuth Chan-ocha would not answer media questions when he arrived at Government House today. The assistant minister at the center of the scandal, Thammanat Prompao, also refused to comment, though he said yesterday he’s planning to sue an Australian newspaper for publishing the claim.

In an interview with BBC Thai, agriculture assistant minister Thammanat also said he has “already explained” everything about the case.

Read: Minister Won’t Explain Heroin Conviction, Jail Time in Australia

An investigative report by the Sydney Morning Herald said Thammanat was sentenced to jail for four years in 1993 over a heroin trafficking conviction. Thammanat previously told local media he served only eight months in jail and was paroled for community service.

Thammanat, whose past careers include a military post, has maintained he was not directly involved in the crime, a claim disputed in detail by the Herald.

The minister dismissed questions from reporters with a wave before entering Government House for a weekly meeting.

The allegation is the latest gaffe to hit the government coalition since coming to power in July. Thai laws bar those convicted of narcotics offenses from serving in the cabinet, but a government official has suggested the restriction is not applicable to politicians prosecuted by foreign courts.

When questioned about the controversy today, PM Prayuth did not give a reply. He later said the media should publish negative news less frequently.

“You often ask questions that turn my mood sour,” the prime minister said as he was touring an exhibition about China’s famed terracotta warriors.

Deputy PM Wissanu Krea-ngam and Interior Affairs Minister Anupong Paochinda said it’s up to Thammanat to explain himself to the public.