WASHINGTON — Former special counsel Robert Mueller says Russia’s efforts to interfere in U.S. elections is “among the most serious” challenges to American democracy.

Mueller made the statement in his opening remarks before the House Judiciary Committee. He’ll appear before the intelligence committee later in the day.

Mueller has expressed his reluctance to testify and said he won’t go beyond what’s in his 448-page report released in April.

Mueller’s report said the investigation did not find sufficient evidence to establish charges of a criminal conspiracy between the Trump presidential campaign and Russia. But it said investigators did not clear President Donald Trump of trying to obstruct the probe.

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