The meeting also was attended by Michael Flynn, Trump’s first national security adviser.

The White House disclosed the meeting only in March, playing down its significance. But people familiar with the matter say the FBI now considers the encounter, as well as another meeting Kushner had with a Russian banker, to be of investigative interest.

Kislyak reportedly was taken aback by the suggestion of allowing an American to use Russian communications gear at its embassy or consulate — a proposal that would have carried security risks for Moscow as well as the Trump team.

[Jared Kushner now a focus in Russia investigation]

Neither the meeting nor the communications of Americans involved were under U.S. surveillance, officials said.

The White House declined to comment. Robert Kelner, a lawyer for Flynn, declined to comment. The Russian Embassy did not respond to requests for comment.

Russia at times feeds false information into communication streams it suspects are monitored as a way of sowing misinformation and confusion among U.S. analysts. But officials said that it’s unclear what Kislyak would have had to gain by falsely characterizing his contacts with Kushner to Moscow, particularly at a time when the Kremlin still saw the prospect of dramatically improved relations with Trump.

Kushner’s apparent interest in establishing a secret channel with Moscow, rather than relying on U.S. government systems, has added to the intrigue surrounding the Trump administration’s relationship with Russia.

The FBI closely monitors the communications of Russian officials in the United States, and it maintains a nearly constant surveillance of its diplomatic facilities. The National Security Agency monitors the communications of Russian officials overseas.

Current and former U.S. intelligence officials said that although Russian diplomats have secure means of communicating with Moscow, Kushner’s apparent request for access to such channels was extraordinary.

“How would he trust that the Russians wouldn’t leak it on their side?” said one former senior intelligence official. The FBI would know that a Trump transition official was going in and out of the embassy, which would cause “a great deal” of concern, he added. The entire idea, he said, “seems extremely naive or absolutely crazy.”

The discussion of a secret channel adds to a broader pattern of efforts by Trump’s closest advisers to obscure their contacts with Russian counterparts. Trump’s first national security adviser, Flynn, was forced to resign after a series of false statements about his conversations with Kislyak. Attorney General Jeff Sessions recused himself from matters related to the Russia investigation after it was revealed that he had failed to disclose his own meetings with Kislyak when asked during congressional testimony about any contact with Russians.

Kushner’s interactions with Russians — including Kislyak and an executive for a Russian bank under U.S. sanctions — were not acknowledged by the White House until they were exposed in media reports.

It is common for senior advisers of a newly elected president to be in contact with foreign leaders and officials. But new administrations are generally cautious in their handling of interactions with Moscow, which U.S. intelligence agencies have accused of waging an unprecedented campaign to interfere in last year’s presidential race and help elect Trump.

Obama administration officials say members of the Trump transition team never approached them about arranging a secure communications channel with their Russian contacts, possibly because of concerns about leaks.

The State Department, the White House National Security Council and U.S. intelligence agencies all have the ability to set up secure communications channels with foreign leaders, though doing so for a transition team would be unusual.

Trump’s advisers were similarly secretive about meetings with leaders from the United Arab Emirates. The Obama White House only learned that the crown prince of Abu Dhabi was flying to New York in December to see Kushner, Flynn and Stephen K. Bannon, another top Trump adviser, because U.S. border agents in the UAE spotted the Emirate leader’s name on a flight manifest.

Russia would also have had reasons of its own to reject such an overture from Kushner. Doing so would require Moscow to expose its most sophisticated communications capabilities — which are likely housed in highly secure locations at diplomatic compounds — to an American.

The Post was first alerted in mid-December to the meeting by an anonymous letter, which said, among other things, that Kushner had talked to Kislyak about setting up the communications channel. This week, officials who reviewed the letter and spoke on the condition of anonymity to discuss sensitive intelligence said the portion about the secret channel was consistent with their understanding of events.

For instance, according to those officials and the letter, Kushner conveyed to the Russians that he was aware that it would be politically sensitive to meet publicly, but it was necessary for the Trump team to be able to continue their communication with Russian government officials.

In addition to their discussion about setting up the communications channel, Kushner, Flynn and Kislyak also talked about arranging a meeting between a representative of Trump and a “Russian contact” in a third country whose name was not identified, according to the anonymous letter.

The Post reported in April that Erik Prince, the former founder of the private security firm Blackwater and an informal adviser to the Trump transition team, met on Jan. 11 — nine days before Trump’s inauguration — in the Seychelles islands in the Indian Ocean with a representative of Russian President Vladimir Putin.