The U.S. Environmental Protection Agency on Friday ordered states to stop blocking contaminated waste from a fiery train derailment in Ohio from being sent to hazardous waste storage sites around the nation.

A handful of politicians and states have sought to block shipments from East Palestine, including Oklahoma Gov. Kevin Stitt, who last week said he had stopped waste from the derailment from coming into his state.

EPA Administrator Michael Regan said there was no reason for states to block shipments of the type of waste that certified facilities routinely handle every day.

“This is impermissible and this is unacceptable,” he said.

In a letter sent to all states, the EPA said that blocking the shipments would likely violate a federal law dealing with the interstate transport of waste as well as the Commerce Clause of the U.S. Constitution, which the agency said limits the power of states to stop the movement of hazardous waste.

Although it’s not clear what steps the EPA might take if states don’t cooperate, the agency told railroad Norfolk Southern on Friday that it expects the company to take legal action if it is unable to ship the waste to certified disposal facilities.

The EPA already ordered Norfolk Southern to cover the costs of cleaning up from the Feb. 3 derailment that toppled 38 rail cars. No one was hurt, but concerns over a potential explosion led state and local officials to approve releasing and burning toxic vinyl chloride from five tanker cars and forced the evacuations of half the village.