This Reuters story was published December 16. Before you say that the statements were not  issued by the agencies, I would say that spies don't generally issue press releases on their activities. Neither the report, nor the quote was denied:

"The FBI backs the CIA's view that Russia intervened to help … Trump win the U.S. presidential election, two senior government officials told Reuters on Friday.

The FBI, CIA and the Office of Director of National Intelligence now all agree that Russia was behind computer hacks into U.S. political institutions during the elections, officials have said. The FBI initially declined to endorse the CIA's conclusion that Russia intervened specifically to help Trump.

The Washington Post earlier on Friday reported that CIA Director … Brennan has circulated a memo seen by U.S. officials and said there was a 'strong consensus among us on the scope, nature and intent of Russian interference in our presidential election.' The FBI had no immediate comment.