Sources told CNN Russian officials bragged in conversations during the presidential campaign that they had cultivated a strong relationship with former Trump adviser retired Gen. Michael Flynn.
They believed they could use him to influence Donald Trump and his team.
Current and former governments officials said, the conversations deeply concerned US intelligence officials, some of whom acted on their own to limit how much sensitive information they shared with Flynn.
He was tapped to become Trump's national security adviser.
One former Obama administration official said, "This was a five-alarm fire from early on, the way the Russians were talking about him.
" Another former administration official said Flynn was viewed as a potential national security problem..