Colombian authorities have captured two leaders of a 20-ton-per-year cocaine operation that sent drug-laden submarines to the world's top drug consumer, the United States, local police said Tuesday.

Colombia, the world's No. 1 cocaine producer, has been wracked for decades by violence that made large swaths of the Andean nation off limits as drug gangs, leftist rebels and right-wing paramilitaries fought for control of a business measured in billions of dollars.

"As a result of an investigation of more than 6 years, the national police ... captured two of those responsible for sending cocaine to North America using self-propelled, semi-submersible vessels, with (shipments) averaging 20 tons of drugs annually," the police said in a statement.

Colombia, where drug traffickers take advantage of dense jungles and forests and a weak state presence in some areas, has received billions of dollars in aid from Washington to fight cocaine output, leftist rebels and cartels.

Colombia's new criminal gangs -- made up of a kaleidoscope of former paramilitary commanders, ex-cartel members and others -- ship tons of cocaine monthly through Central America and Mexico to the United States and to a lesser extent to Europe.

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