A marijuana plant grows at the Colorado Harvest Company in Denver, Colo. on Wednesday, July 8, 2015.

(Nathaniel Minor/CPR News)

Nebraska Attorney General Doug Peterson and his counterpart in Oklahoma are joining a lawsuit aimed at halting legal marijuana in Colorado.

The Lincoln Journal Star reports that the two states asked to be added as plaintiffs this month in a case being considered by an appeals court in Denver.

The appeal combines two separate cases: one on behalf of a Colorado couple who own land near a recreational marijuana growing facility and another brought one by a group of sheriffs from Colorado, Kansas and Nebraska.

Oklahoma and Nebraska argue they have "unique sovereign interests" in stopping marijuana from crossing their state borders, and that they shouldn't be left out as the court weighs the issue.

"Because the people of Nebraska and Oklahoma have determined that marijuana is harmful and should be illegal, Nebraska and Oklahoma have a duty to protect their citizens from the continuing harms resulting from Colorado's illegal activities, by taking action to ensure that Colorado marijuana does not enter their sovereign boundaries," the states' lawyers wrote.

The U.S. Supreme Court denied considering a similar lawsuit by states in March.