Security sources told the Standard daily that the 442-dwt Amin Darya (built 1987, also known as Al Noor) was held two weeks ago.
Anti-narcotics officers have now succeeded in extracting 341.7kg of heroin with a street value of $12.53m after initially discovering 900 grams hidden in a cement cargo.
Hamisi Massa, head of the anti-narcotics unit, told the All Africa news agency: "Upon inspection our officer stumbled on narcotics poured in the diesel tank.
