Drug traffickers are moving to Colorado to grow marijuana and ship it to other states, the Associated Press reports. Their findings come from interviews with law enforcement officials and a review of court records.

Opponents of marijuana legalization have long said Colorado’s move to legalize recreational marijuana would lead to more illegal trafficking to other states. Traffickers are growing marijuana amid state-sanctioned warehouses and farms, secretly shipping it out of state and making millions of dollars, according to AP.

In one instance, the owner of a skydiving business filled his planes with hundreds of pounds of marijuana grown in Colorado, and flew them to Minnesota. Associates allegedly sold the marijuana for millions of dollars in cash. In another case, a Denver man was charged with sending more than 100 FedEx packages stuffed with marijuana to Buffalo, New York. Twenty traffickers were accused of relocating to Colorado to grow marijuana and send it to Florida, where it can be sold for more than double the price charged by legal Colorado shops.

Last year Nebraska and Oklahoma filed a lawsuit against Colorado with the U.S. Supreme Court, saying the state’s law legalizing recreational marijuana is unconstitutional and places a burden on them. The states said they are having trouble protecting their borders from the increased flow of marijuana, and asked the court to block Colorado’s legal marijuana system.

In December the federal government advised the Supreme Court to avoid weighing in on the lawsuit.

Many illegal marijuana growers come from out of state, never get a growing license and “don’t even attempt to adhere to the law,” said Barbra M. Roach, Special Agent in charge of the Drug Enforcement Administration’s Denver field division. “It’s like hiding in plain sight,” she said.