WASHINGTON -- Law enforcement sources tell CBS News it is unlikely former National Security Adviser Michael Flynn will face charges in connection to his discussions of U.S. sanctions with the Russian ambassador to the U.S.

In late December, routine surveillance of Russian officials and operatives picked up Flynn’s voice on a call with Sergey Kislyak.

Sources say transcripts showed the two discussed Obama administration sanctions on Russia, a possible violation of federal law since President Trump was not yet in office.

In the early days of the Trump administration, Flynn was interviewed by FBI agents about the call. He also told Vice President Pence that he did not discuss sanctions with Kislyak, a claim the vice president repeated on television.

But investigators say that despite misleading the vice president, there is not enough evidence to charge him with lying to the FBI.

Flynn has also not been cleared in the broader investigation into whether he and others in the Trump campaign were in regular contact with Russian officials during the election.

The U.S. intelligence community has concluded that Russia carried out cyberattacks on Democratic Party officials designed to undermine Hillary Clinton.

On Thursday, the president denied he or any of his aides were involved.

“Russia is a ruse. I know you have to get up and ask your question but Russia is a ruse. I have nothing to do with Russia, haven’t made a phone call to Russia in years,” he said.

On Friday, members of the Senate Intelligence Committee were briefed on the investigation by FBI Director James Comey.

Sources say getting to the end of this investigation won’t be a sprint; it’s a marathon and there’s still a ways to go.