But before tribal police could catch up with two semi-trucks on federal highways, they learned the vehicles under contract with Energy Fuels Inc. no longer were on the reservation.

Navajo President Buu Nygren vowed to carry out the plan to enact roadblocks while the tribe develops regulations over the first major shipments of uranium ore through the reservation in years.

“Obviously the higher courts are going to have to tell us who is right and who is wrong,” he told The Associated Press. “But in the meantime, you’re in the boundaries of the Navajo Nation.”

The tribe passed a law in 2012 to ban the transportation of uranium on the vast reservation that extends into Arizona, New Mexico and Utah. But the law exempts state and federal highways that Energy Fuels has designated as hauling routes between the Pinyon Plain Mine south of Grand Canyon National Park for processing in Blanding, Utah.

Still, Nygren and Navajo Attorney General Ethel Branch believe the tribe is on solid legal footing with a plan for police to block federal highways, pull over drivers and prevent them from traveling farther onto the reservation.

Energy Fuels said it began hauling the ore as planned Tuesday and had informed federal, state, county and tribal officials more than 10 days ago about the legal requirements, safety, emergency response and the imminent shipping of uranium ore, though it didn’t give a specific date. Spokesman Curtis Moore said no one said that wasn’t sufficient.

“Tens of thousands of thousands of trucks have safely transported uranium ore across northern Arizona since the 1980s with no adverse health or environmental effects,” the company’s president and chief executive, Mark Chalmers, said in a statement. ”Materials with far greater danger are transported every day on every road in the county. Ore is simply natural rock. It won’t explode, ignite, burn or glow, contrary to what opponents claim.”

The Arizona Department of Transportation and the Arizona Department of Public Safety, which have jurisdiction on state and federal highways through the reservation, didn’t immediately return email messages seeking comment.

The Kaibab National Forest where the mine is permitted said it was notified after hauling began Tuesday morning, then contacted tribes, local officials and others, spokeswoman Brienne Pettit said. The forest also requested advanced notification, she said.

Officials with Coconino County and the Navajo Nation said Energy Fuels verbally agreed — but is not required to — give communities along the route at least a week’s notice before any truck hauled uranium through them. Nygren said the tribe got a notification Tuesday that trucks had left the mine site and were driving north through Flagstaff.

Energy Fuels, the largest uranium producer in the United States, recently started mining at the Pinyon Plain Mine for the first time since the 1980s, driven by higher uranium prices and global instability. The industry says uranium production is different now than decades ago when the country was racing to build up its nuclear arsenal.

No other sites are actively mining uranium in Arizona.

Chalmers said uranium ore mined from Pinyon Plain is transported in tightly covered vehicles that have placards indicating the contents are radioactive. The shipments, driven by licensed and trained operators, comply with regulations and law, he said. Mining during World War II and the Cold War left a legacy of death, disease and contamination on the Navajo Nation and in other communities across the country, making any new development of the ore a hard pill to swallow. The Havasupai tribe is among the tribes and environmentalists that have raised concerns about potential water contamination.

Republicans have touted the economic benefits the jobs would bring to the region known for high-grade uranium ore.

In 2013, the Navajo Nation told another uranium producer that it would deny access to a ranch that surrounded a parcel of Arizona state trust land where the company planned to mine. At the time, the tribe cited a 2005 law that banned uranium mining on its lands and another 2006 law that addressed transport. The mining never occurred, although it also needed other things like a mineral lease and environmental permits.

Stephen Etsitty, executive director of the Navajo Nation Environmental Protection Agency, said the tribe had been meeting with Energy Fuels since March to coordinate emergency preparedness plans and enact courtesy notifications.

Based on those meetings, Etsitty said the tribe didn’t expect Energy Fuels to transport uranium through the Navajo reservation for at least another month or until the fall.

On Tuesday, he said the tribe found out indirectly about the trucks, leaving officials frustrated on what is primary election day in Arizona. Moore, the company spokesman, said Energy Fuels is required to notify the tribe of any spills or accidents.

Etsitty said accidents involving trucks carrying hazardous or radioactive material occur on average once every three to five years on the reservation. But the possibility requires the tribe to notify emergency responders along the route. Because the material being transported from the mine is uranium ore, rather than processed ore, the risk of radiation exposure is lower, Etsitty said.

“It is a danger, but it would take a longer period of time for somebody to get acute exposure at a spill site,” he said. “Precautions still need to be taken.”