Andrew Harnik/AP

Former special counsel Robert Mueller said Russia's interference in the 2016 presidential election was "not a hoax."

His remarks on Russian interference came during a line of questioning by Rep. Jackie Speier, who described it as "an invasion" as well as "sinister and scheming."

Asked if he would agree that it was not a hoax and that the Russians were engaged in trying to impact the US election, Mueller said, "Absolutely, it was not a hoax."

"The indictments we returned against the Russians — two different ones — were substantial in their scope, using the scope word again. And I think we have underplayed to a certain extent that aspect of our investigation that has and would have long term damage to the United States that we need to move quickly to address.”