But what is also peculiar is the level of trust Kushner would have been placing in Russian officials in asking for such a communications channel. Foreign affairs is often complex, yet Kushner didn’t want the U.S. government’s help—or supervision.

"What is unusual and borderline disturbing about this is less that it cut out the State Department or cut out the intelligence community; I think there is a precedent for both of those things in back-channels," said Jon Finer, former State Department chief of staff under John Kerry. “It shows a level of trust in Russian intelligence, and Russian diplomatic personnel beyond the level of trust afforded to American intelligence and American personnel.”

The White House has obliquely defended Kushner’s actions while refusing to comment on them specifically. “We have back-channel communications with a number of countries. So, generally speaking, about back-channel communications, what that allows you to do is to communicate in a discreet manner,” National-Security Adviser H.R. McMaster told reporters on Saturday. Asked whether it would be cause for concern if a National Security Council staffer used such a back-channel to Moscow, he said: "No, I would not be concerned about it."

"What puts this in an entirely different category is that this is a transition; they weren't in the government yet,” said Paul Pillar, a former analyst with the Central Intelligence Agency. "That's really a departure. It's normal for an incoming administration to have contacts with foreign leaders, but I can't think of a precedent for this kind of thing."

And former national-security officials noted that while back-channel communications are often compartmentalized—meaning they can only be viewed by a select number of officials—they usually have some level of involvement from national-security officials. Communicating with Moscow using Russian facilities could have shielded Kushner’s correspondence from U.S. intelligence agencies, without denying their Russian counterparts the same access.

“The only reason you would operate that way is if you were hiding something from your own government. That's it. That's the only plausible explanation," said Nada Bakos, a fellow at the Foreign Policy Research Institute and a former CIA analyst. “There's compartmentalized classification—if they wanted to take this to the highest level of classification they could do that. It didn't have to be widely disseminated.”

Reports from The New York Times and The Wall Street Journal have cited anonymous administration officials claiming that the purpose of the communications would have been to discuss the Syrian civil war. But that explanation raises similar issues: If that was the topic, why would Kushner want to cut out U.S. officials? And why couldn’t it wait until after the transition?