Washington has promised action against Mexican drug-traffickers who the US says are growing billions of dollars of marijuana in US national parks.
Mexican cartels controlled 80% of the marijuana grown in forest and mountain areas, drugs tsar John Walters said.
He blamed the cartels for much of the expansion of the crop, in particular in public lands such as national parks.
Every American should be outraged that national parks were being turned into centres of drug production, he said.
But growing marijuana in the US is not entirely a new phenomenon, despite it being illegal for more than 70 years.
According to the National Organisation for the Reform of Marijuana Laws (Norml), back in 1998 US-grown marijuana was thought to be among the top five cash crops in the country, with an estimated value of over $10bn (£5.3bn).
Five years later, it was estimated that the value of US marijuana crops could be as high as $25bn.
That figure, if correct, would make it more valuable that the country's largest legal cash crop, which is maize and produces about $19bn in revenue.
Traditionally, most of the trade has taken place in California because of its warm climate, but now American experts say it is expanding rapidly to other states in the north and east and even to Hawaii.
Perhaps more worrying, much of this expansion has been on public lands.
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