Sen. Bill Nelson, a Florida Democrat, has reaped the political whirlwind in the 10 days since he proclaimed that Russian hackers had “penetrated” some of his state’s county voting systems.

The Republican governor of Florida, who happens to be running against Nelson for his U.S. Senate seat this fall, has blasted his claim as irresponsible. The top Florida elections official, also a Republican, said he had seen no indication it’s true. And the Washington Post weighed in Friday with a 2,717-word fact check that all but accused Nelson — without evidence — of making it up.

However, three people familiar with the intelligence tell NBC News that there is a classified basis for Nelson’s assertion, which he made at a public event after being given information from the leaders of the Senate Intelligence Committee. The extent and seriousness of the threat remains unclear, shrouded for reasons of national security.