Law enforcement officials say Mexican drug gangs are quietly commandeering U.S. public land to grow huge marijuana crops and using smuggled immigrants to cultivate them.

Some of traffickers have planted pot fields near Yosemite's waterfalls and in the middle of California's redwood forests.

Interviews conducted by The Associated Press with law enforcement officials across the country showed that Mexican gangs are mostly responsible for a spike in large-scale marijuana farms over the last several years.

Americans have been growing pot on government land for years, but authorities say Mexican gangs have taken it to a whole new level. Many of them use armed guards and trip wires to safeguard sprawling farms containing tens of thousands of plants.