Sen. John McCain, R-Ariz., speaks with employees at Robertson Fuel Systems earlier this year in Tempe, Ariz.  (AP Photo/Ross D. Franklin)

Sen. John McCain blasted the company that lifts national security satellites to space, saying it has made “troubling and specious claims” about its sudden inability to participate in a recent competition to launch an Air Force satellite.

In a letter sent to defense Secretary Ashton Carter Tuesday, McCain (R-Ariz.) once again took aim at the United Launch Alliance, the joint venture between Lockheed Martin and Boeing, which for a decade held a monopoly on Pentagon and intelligence community launches.

Last month, ULA announced that it could not bid on the upcoming contract—the first time it would have faced competition—for a variety of reasons. That means that SpaceX, the only other company certified by the Pentagon to launch payloads for national security agencies, would walk away with the lucrative contract.

One of the reasons that ULA said prevented it from bidding on the contract was a congressionally-mandated limit on the Russian-made engine that powers its Atlas V rocket. Congress imposed the limit in response to the escalating tension with Russian over that nation’s actions in Ukraine.

ULA has said the RD-180s it has in stock have been assigned to other missions and were no longer available for Pentagon launches.

[ULA bows out of Pentagon launch competition, paving way for SpaceX]

But McCain said he found that claim “especially dubious.”

“Instead of setting those engines aside for national security launches, ULA rushed to assign them to non-national security launches that are unrestricted in their use of Russian engines,” McCain wrote.

He accused ULA of trying to artificially create a need for relief from the restrictions and “subvert the authorization process.”

A spokesperson for ULA declined to comment Wednesday.

[ULA under pressure from Elon Musk’s SpaceX upstart and Congress]

For years, ULA had a monopoly on the launches, a lucrative and steady source of revenue for two of the biggest defense contractors in the world. Then SpaceX came along, and the hard-charging company founded by tech billionaire Elon Musk said it could provide the launches for far less.

Musk eventually sued the Air Force, saying that SpaceX should be allowed to compete for the business. Ultimately, the parties settled, and the Air Force certified the company’s Falcon 9 rocket for the launches. The Air Force contract would have been the first time that SpaceX would have challenged ULA.

In a statement last month, ULA said “it wants nothing more to compete,” but was prevented from doing so because of the lack of engines, and because it could not comply with the accounting structures required under the contract. It also said that the Air Force used a procurement process that would give a lot of weight to the prices companies bid — and not their experience and past performance, which could have given ULA an edge.

But McCain said the assertion that it’s a “low-price” contract “is erroneous.”

Rather the contract is a “best value” source selection that calls for “a careful evaluation of performance, launch operations, schedule and price,” he wrote.

ULA has said it remains hopeful that Congress would eventually loosen the restrictions on the RD-180 engine. And Sen. Richard Shelby (R-Ala.) has said he is considering proposing language that would do just that.

After Shelby’s announcement, McCain fired off a letter last month to Sen. Thad Cochran (R-Miss), the Appropriations Committee chairman, that such a move “would undermine sanctions on Russian rocket engines.”

“Recent attempts by the incumbent contractor to manufacture a crisis by prematurely diminishing its stockpile of engines purchased prior to the Russian invasion of Crimea should be viewed with skepticism and scrutinized heavily,” he wrote.