“There has as yet been no such determination made with respect to the press reports in questions,” a State Department official told the Washington Examiner.

Those internal deliberations take place in a difficult context, with respect to policy and politics. An administration decision not to sanction China could undermine its ability to discourage allies such as Turkey or India from making similar purchases. A decision against sanctions might also weaken the anti-Russia measures, and prompt a political backlash from congressional Democrats.

Then there’s the technical, legal question: does the Russia sanctions law — known formally as the Countering America’s Adversaries Through Sanctions Act — cover the air defense purchase? Some key lawmakers disagree, in part because the sale was made years before the law passed.

“If they entered an agreement to purchase the equipment prior to CAATSA being passed, there’s a likelihood it would not be counted as an infraction,” Senate Foreign Relations Chairman Bob Corker, R-Tenn., told the Washington Examiner. “It’s difficult to go back on previous commitments.”

Corker’s Democratic counterpart on the committee disagrees. “That would be a sanctionable activity,” Sen. Bob Menendez, D-N.J., said when asked about China.

The divergence stems, in part, from the ambiguity of the CAATSA law, which contemplates sanctions for “significant transactions” but doesn’t define the term. An S-400 missile system is certainly significant; the weaponry would allow China to target fighter jets flying in contested airspace, perhaps most notably Taiwan.

“The State Department is robustly implementing CAATSA,” the official told the Washington Examiner. “[T]he significance of a transaction is determined on a case-by-case basis using a multifactor analysis that considers all relevant elements, such as, but not limited to, the size, scope, value to Russia, and sophistication of the transaction, and its impact on U.S. national security and foreign policy interests.”

But is the transaction completed when the contracts are signed, or at the time of payment, or of delivery? Lawmakers aren’t sure, and the administration isn’t saying.

“It’s a question of whether you actually take possession of the S-400s … actually deploy it — from my view, you’re sanctionable,” Menendez said.

China has received “the first regimental set” of missiles, but their deployment was delayed after a storm forced a ship carrying some of the missile systems to return home.

“The work continues, and the missing equipment is expected to be delivered to the customer in the summer,” TASS, a Russian state-run media outlet, quoted a “military-diplomatic source” as saying.

China’s purchase of S-400s has been overshadowed, in U.S. circles, by Turkey’s pursuit of the same weapons system. That deal would place an advanced Russian anti-aircraft weapons system into a NATO country, thereby heightening the risk of espionage against the alliance and undermining military cooperation. The State Department has warned Turkey that the deal, if completed, will provoke sanctions. Still, that decision would not set a perfect precedent for China, because of the timing of the deal.

“I think at least the State Department was looking into when they actually placed the order,” Corker explained. “My guess is that they were not ordered before the sanctions went in place”

If the administration does impose the sanctions, that would be another irritant to a U.S.-China relationship already fraught by a potential trade war, the need for unity in response to North Korea’s nuclear weapons and ballistic missile program, and China’s aggressive assertion of sovereignty over key international shipping lanes. Those factors shouldn’t stop the State Department, though, Menendez argued.

“Let’s say for argument’s sake they are sanctionable: if you don’t sanction them, then the message is that you can ultimately purchase and deploy an S-400 and the consequences are — none,” he told the Washington Examiner. “And if you try to differentiate between China, because of it’s size, and a Turkey, because it may be smaller than a China, then you have a slippery slope.”

In any case, CAATSA gives the administration a range of sanctions that it can choose to apply, while stopping short of the most aggressive diplomatic and economic punishments.

“They aren’t what we call blocking sanctions which are visa bans and asset freezes — thats the toughest brand of sanctions that can be applied and so those aren’t what that menu is,” a Senate aide familiar with the law explained. “Depending on what they choose, it could have a tough impact or it may not have much of an impact.”

If the administration declines to impose the sanctions, Democrats are likely to regard it as another example of Trump hesitating to confront Russia over the 2016 cyber attacks against the Democratic party and presidential nominee Hillary Clinton’s campaign.

“The administration has generally been loathe to sanction entities as called for under CAATSA, including mandatory sanctions,” Menendez told the Washington Examiner. “And so, that would be extremely concerning.”

If the State Department concludes that the law does not mandate sanctions over the purchase air defense systems, that might only delay a confrontation with China over deals with Russia’s defense industry.

“Russia and China plan to sign a contract on production of the Advanced Heavy Lifter (AHL) helicopter in May-June,” TASS reported Wednesday.