To prove a vast conspiracy involving Vietnam's most feared and powerful institution - the Interior Ministry - prosecutors have started to focus on the contents of a humble fuel tank, court sources said. Among allegations he made on death row, heroin dealer Sieng Pheng said police initially only found about half of the 31 kilograms of the drug he had hidden in his car when he was arrested. Those officers allegedly included his long-time partner, Truong, a fast-rising federal police officer. Pheng held his silence until the day he was due to be executed. He alleged Truong later arranged for the car to be sold after cleaning out more than 15 kg from the fuel tank. He had doctored investigations, sparking a cover-up that swept the ministry. Truong was arrested last July at one of his three houses. Hanoi police found the missing heroin, two pistols and nearly US$100,000 (HK$773,000), a considerable sum for an officer whose pay was about HK$50 a month. The haul was part of an estimated 300 kg which a syndicate, said to involve Truong and Pheng, smuggled in police vehicles from the Golden Triangle through Laos to Vietnam and beyond. Court sources said the pair faced each other in silence yesterday at the start of 10 days of closed-door hearings, with Pheng's fate unknown after an unprecedented reprieve from President General Le Duc Anh. Why Truong would try to cheat his 'fellow trafficker' remains a key question. Some sources say Truong - known as 'The Bald One' - became too greedy after being posted from Lai Chau in Vietnam's isolated northwest to a senior position in the national anti-social evils squad. Others say he was always criminally ambitious.