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Nebraska and Oklahoma challenged neighboring Colorado’s recreational marijuana laws in the U.S. Supreme Court on Thursday amid complaints its pot was seeping across their borders, and Colorado vowed to defend its laws.

Nebraska Attorney General Jon Bruning said he joined Oklahoma in filing the action against Colorado, where voters chose to legalize recreational marijuana in a landmark 2012 vote even as the drug remains federally outlawed.

“Federal law undisputedly prohibits the production and sale of marijuana,” Bruning said in a statement, adding that drugs threaten the health and safety of children, and calling narcotics trafficking a national, interstate problem.

The lawsuit accuses Colorado of creating “a dangerous gap” in the federal drug control system.

“Marijuana flows from this gap into neighboring states, undermining plaintiffs states’ own marijuana bans, draining their treasuries, and placing stress on their criminal justice systems,” the lawsuit said.

Colorado Attorney General John Suthers responded that because neighboring states had expressed concern about Colorado-grown pot crossing their borders, he was “not entirely surprised” by the legal challenge.

“However, it appears the plaintiffs’ primary grievance […]