The U.S. official in charge of protecting American elections from hacking says the Russians successfully penetrated the voter registration rolls of several U.S. states prior to the 2016 presidential election.

In an exclusive interview with NBC News, Jeanette Manfra, the head of cybersecurity at the Department of Homeland Security, said she couldn't talk about classified information publicly, but in 2016, "We saw a targeting of 21 states and an exceptionally small number of them were actually successfully penetrated."

Jeh Johnson, who was DHS secretary during the Russian intrusions, said, "2016 was a wake-up call and now it's incumbent upon states and the Feds to do something about it before our democracy is attacked again."