Here is the bone of contention, Mueller did not say it was not true, only that it was inaccurate. It was such an important political issue that they had to clarify things somewhat, Buzzfeed stands by the story.
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https://www.cnn.com/2019/01/18/politics/mueller-statement-buzzfeed/index.html
(CNN)Special counsel Robert Mueller's office disputed an explosive story from BuzzFeed News as
"not accurate" Friday night, after the news outlet reported the President had directed his personal attorney Michael Cohen to lie to Congress, for which Cohen was later prosecuted.
"BuzzFeed's description of specific statements to the Special Counsel's Office, and characterization of documents and testimony obtained by this office, regarding Michael Cohen's Congressional testimony are not accurate," said Peter Carr, a spokesman for Mueller's office, in a statement.
It's highly unusual for the special counsel's office to provide a statement to the media -- outside of court filings and judicial hearings -- about any of its ongoing investigative activities.
In response, BuzzFeed said in its own statement, "We are continuing to report and determine what the special counsel is disputing. We remain confident in the accuracy of our report."
But following the story's publication late Thursday night, Democratic members of Congress began pointing to the report as grounds for the President's impeachment. The clamor grew throughout the day and into Friday night.