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The U.S. Supreme Court declined on Monday to take up a lawsuit filed by two of Colorado's neighboring states over its legalization of marijuana.

Nebraska and Oklahoma said Colorado's decriminalization has "increased the flow of marijuana over their borders," forcing them to expend greater "law enforcement, judicial system, and penal system resources," thereby harming the welfare of their residents.

Monica Lo, co-founder of Asian Americans for Cannabis Education, holds a marijuana leaf. Courtesy of Monica Lo

They claimed to suffer "a direct and significant detrimental impact — namely the diversion of limited manpower and resources to arrest and process suspected and convicted felons involved in the increased illegal marijuana trafficking or transportation."

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Colorado's approach, they argued, is in direct conflict with federal law, which makes it illegal to possess even small amounts of marijuana.

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The court turned the case away in an unsigned opinion. Justices Clarence Thomas and Samuel Alito dissented. Writing for them both, Thomas said court should have taken the case because "the plaintiff states have made a reasonable case."

There were no lower court decisions, because disputes between the states come directly to the Supreme Court.

Nebraska and Oklahoma did not challenge Colorado's legalization itself. Instead, they said the way it regulates the manufacture, possession, and distribution of marijuana was causing them harm.

Three anti-abortion protestors are seen outside U.S. Supreme Court building is seen in Washington, March 16, 2016. President Barack Obama will nominate Merrick Garland today to fill the position of deceased Court Justice Antonin Scalia. REUTERS/Jim Bourg JIM BOURG / Reuters

But Colorado said its neighboring states real quarrel is with the federal government's policy of declining to prosecute cases of simple possession in states where marijuana use has been legalized. Those states should sue the federal government, not Colorado, it said.

"A state does not violate the sovereign rights of another state, by making a policy decision that parts ways with its neighbors," Colorado said.

The Obama Justice Department urged the Supreme Court not to take the case. "Entertaining the type of dispute at issue here — essentially that one state's laws make it more likely that third parties will violate federal and state law in another state - would represent a substantial and unwarranted expansion of" of the Supreme Court's jurisdiction.