Investigators believe that President Trump’s national security adviser, Michael Flynn, privately discussed U.S. sanctions against Russia in a phone call with a Russian official, law enforcement sources told CBS News on Friday.

Multiple sources told CBS News’ Jeff Pegues and Pat Milton that the conversation occurred before Mr. Trump took office and, if true, could be a violation of protocol and could be viewed as a violation of the law.

A law enforcement source who has been briefed on the issue told Milton that the discussion dealt with the relationship going forward with Russia including the sanctions. Any discussions about sanctions by a private citizen, the source said, may create conflict and confusion around U.S. national security interests.

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The sources told CBS News that investigators learned of the discussions through continuing and ongoing electronic surveillance of Russian officials as well as known and suspected intelligence operatives in the U.S.

The Washington Post first reported about the discussions late Thursday, saying that the discussions were between Flynn and Russian Ambassador Sergey Kislyak. When asked in an interview with the Post Wednesday if he had discussed sanctions with the ambassador, Flynn repeatedly said, “No.”

On Thursday, however, Flynn appeared to retreat from that denial. His spokesman told the Post, that Flynn “indicated that while he had no recollection of discussing sanctions, he couldn’t be certain that the topic never came up.”

Trump transition and administration officials have acknowledged that there were phone calls between Flynn and Kislyak. At least one of the calls was made in late December, around the time the Obama Administration was expelling dozens of Russian diplomats. At the time, President Obama had announced sanctions in response to cyberattacks.

A congressional source told CBS News there is no evidence that General Flynn made a specific promise or that he signaled pending policy changes to the ambassador. It appears the topic of sanctions came up in an incidental, off-hand way and the general dismissed discussing until later. There is concern about yet another potential leak of very sensitive information. The source said the leaks all go in the same direction: Trying to discredit President Trump, his administration and the people around him.

Vice President Mike Pence told CBS’s “Face the Nation” last month that there was no contact between the Trump team and Russian officials during the campaign and Flynn’s call with Kislyak on the same day sanctions were announced in December was “strictly coincidental.”

“Of course not,” Pence said on the program when asked whether anyone from the campaign was ever in touch with Russia. “I think to suggest that is to give credence to some of these bizarre rumors that have swirled around the candidacy.”

“They did not discuss anything having to do with the United States’ decision to expel diplomats or impose censure against Russia,” Pence said.

One source in Pence’s circle told CBS News Friday that what Pence said on “Face the Nation” about Flynn’s contact with the Russian ambassador was based on what Flynn told him directly about the phone calls.

At a daily White House press briefing last month, press secretary Sean Spicer claimed that Flynn had two calls on four subjects with the ambassador including Christmas greetings and arranging a phone call between Mr. Trump and Russian President Vladimir Putin.

CBS News’ Jeff Pegues, Pat Milton and Steven Portnoy contributed to this report.