The FBI in late December reviewed intercepts of communications between the Russian ambassador to the United States and retired Lt. Gen. Michael T. Flynn — national security adviser to then-President-elect Trump — but has not found any evidence of wrongdoing or illicit ties to the Russian government, U.S. officials said.

The calls were picked up as part of routine electronic surveillance of Russian officials and agents in the United States, which is one of the FBI’s responsibilities, according to the U.S. officials, who spoke on the condition of anonymity to discuss counterintelligence operations.

Nonetheless, the fact that communications by a senior member of Trump’s national security team have been under scrutiny points up the challenge facing the intelligence community as it continues its wide-ranging probe of Russian government influence in the U.S. election and whether there was any improper back-channel contacts between Moscow and Trump associates and acquaintances.

[U.S. counterintelligence officials are examining possible ties between Russia and Trump associates]

Although Flynn’s contacts with Russian Ambassador Sergey Kislyak were listened to, Flynn himself is not the active target of an investigation, U.S. officials said. The Wall Street Journal reported Sunday that U.S. counterintelligence agents had investigated the communications between Flynn and Kislyak.

(Peter Stevenson/The Washington Post)

Of particular note was a Dec. 29 telephone conversation, initiated in an exchange of text messages the day before. Trump officials previously had said the call took place on the 28th. On the 29th, the Obama administration announced sanctions against Russia and expelled 35 officials from the Russian Embassy in response to what the U.S. intelligence community has said was interference in the presidential election on Trump’s behalf.

Earlier this month and on Monday, during his first official White House news conference, press secretary Sean Spicer said that the call covered several subjects. They included a Russian invitation to the Trump administration to take part in Russian-sponsored Syrian peace talks that began Monday in Kazakhstan. The men also talked about logistics for a post-inauguration call between Trump and Russian President Vladi­mir Putin.

Flynn also conveyed condolences for a Russian plane crash that killed a famed military band the day before the call, said Spicer, who said that Kislyak initiated the call after he and Flynn exchanged holiday greetings by text. Spicer also said Monday that the two had followed up with a subsequent call “two days ago . . . three days ago” to further discuss a Trump-Putin call.

In remarks when the Dec. 28 call was first reported this month, Spicer and other officials said there had been no mention of the sanctions that were announced the next day. On Monday, he said he was unaware of any other conversations between Flynn and members of the Russian government. Spicer said he asked Flynn if there had been conversations with any other Russian officials “beyond the ambassador. He said no.”

Earlier news media reports had also cited a Flynn call to Kislyak on Dec. 19 to express condolences for the terrorist killing of the Russian ambassador to Turkey that day.

Although Flynn has written critically about Russia, he also was paid to deliver a speech at a 2015 Moscow gala for RT, the Kremlin-sponsored international television station, at which he was seated next to Putin.

The FBI’s counterintelligence agents listen to calls all the time that do not pertain to any open investigation, current and former law enforcement officials said. Often, said one former official, “they’re just monitoring the other [foreign official] side of the call.”

Dmitry Medvedev , the prime minister of Russia, walks with Sergey Kislyak, Russian ambassador to the U.S., as he arrives for the G8 Summit at Dulles International Airport in Chantilly, Va., May 18, 2012. (Joshua Roberts/Reuters)

Both Flynn, a former head of the Pentagon’s intelligence agency, and Kislyak, a seasoned diplomat, are probably aware that Kislyak’s phone calls and texts are being monitored, current and former officials said. That would make it highly unlikely, the individuals said, that the men would allow their calls to be conduits of illegal coordination.

greg.miller@washpost.com

Karen DeYoung and David Nakamura contributed to this report.

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