The Department of Energy (DOE) is pushing back on Gov. Steve Sisolak’s response to revelations the department shipped weapons-grade plutonium to Nevada last year, claiming state and congressional leaders were aware of the agency’s plans.

A senior DOE official told Fox News he “personally flew out to Nevada in late July and met with (then-Gov. Brian Sandoval’s office and the state's Dept. of Conservation and Natural Resources) face-to-face to walk them through a number of issues, including this one, in terms of what we were doing relative to plutonium shipments.”

The official added, “What we disclosed to them was the approximate amount of material we were going to ship, which was one metric ton, and we told them it will all be done by 2019. In a report we sent to Congress, we told Congress that half of it would be done by the end of 2018."

The Justice Department notified a federal judge in Reno on Wednesday that the government trucked in the radioactive material to store at a site 70 miles north of Las Vegas sometime last November, before Nevada asked a court to block the move.

Department lawyers said in a nine-page filing that the previously classified information about the shipment from South Carolina -- where a federal judge had ordered that the plutonium be removed from a Savannah River site by 2020 -- could be disclosed because enough time passed to protect national security.

“This material is not waste. We could not have been more clear in our communications with them that this material is not waste and it is the same material we use in our national security missions across the board,” the official added.

Sisolak, a Democrat, condemned the revelation of shipments during a news conference Wednesday afternoon, describing the move as “completely unacceptable deception from the U.S. Department of Energy.”

He also said in a statement, “The Department led the State of Nevada to believe that they were engaging in good-faith negotiations with us regarding a potential shipment of weapons-grade plutonium, only to reveal that those negotiations were a sham all along.”

However, the DOE said it had been “very clear and transparent throughout the entire process,” adding that in addition to speaking with the office of Sandoval, a Republican, the agency conducted talks with then-Sen. Dean Heller, also a Republican, and provided email notification in August regarding the shipments to Democratic Sen. Catherine Cortez Masto.

Cortez Masto did not immediately reply to Fox News’ request for comment regarding the plutonium shipments and whether she was notified about the agency’s intentions ahead of time.

She did say in a statement that she met with officials from the National Nuclear Security Administration (NNSA) Thursday afternoon to express her disapproval.

“The Department of Energy (DOE) and NNSA negotiated in bad faith, hiding the timing of their shipment and refused to share crucial information with Members of Congress who had the security clearance to know. I put NNSA officials on notice that they had betrayed the trust of Nevadans and completely undermined a bipartisan working relationship established by Governor Sandoval, and continued by Governor Sisolak,” Cortez Masto said. “While I remain hopeful we can rebuild that relationship, I made it clear that I’m fighting alongside Nevada’s congressional delegation, Attorney General (Aaron) Ford, and our Governor to hold them accountable and to find out when this plutonium will be removed.”

Immediate attempts to reach Heller and Sandoval were unsuccessful.

A source speaking for Sisolak’s office acknowledged that the DOE sent the shipment ahead of the state’s preliminary injunction that was filed in December, but told Fox News the issue stemmed from being led on “throughout the negotiations, throughout this process with the injunction that no shipment had taken place when... all along there had in fact been a shipment.”

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The source continued, “The DOE did not disclose that to us and they did not disclose that to the federal judge,” adding that the main point of disagreement was with the supplemental analysis from the Agency released in August that "did not allow the state of Nevada to assess the risks posed by shipping this material and to prepare adequately in case of a mishap or accident along the way."

The radioactive material in question is routinely shipped to Nevada, among other states, and “is always done with the locations and the routes and the timing as secret,” the DOE official said.

Nevada has long expressed opposition toward being a nuclear dump site for the nation's radioactive material, with Yucca Mountain remaining a critical point of contention between state officials and the federal government.

James Acton, co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace, called the issue of storing the country's plutonium and nuclear waste a "terrible situation" for everybody.

"There are no good answers in the short term about what to do with this. I mean, of course you understand from a Nevadan perspective... it is a dangerous material. Of course you understand from their perspective why they don't want it," Acton told Fox News.

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He said the issue originated from a much larger problem: "We as a nation never developed a way of getting rid of all this plutonium that we produced."

Acton added, "I would look at this in terms of a national problem that's an impossible situation... Nevada has the facilities in the country where this stuff can be stored -- so they're kind of screwed over by it. As I say, I don't know what else DOE was supposed to do in this situation. It's just an impossible situation."

The Associated Press contributed to this report.