Jeh Johnson told lawmakers Thursday he's unaware of any evidence showing cyberattacks affected voting in the 2016 presidential election.

But, the former secretary of homeland security said it's a concerning future possibility.

"The integrity of our election outcomes on a national level dances on the head of a pin," he told an election security task force assembled by congressional Democrats. "If writers of the TV series ‘House of Cards' can figure that out, then a lot of other people can do the same."

Johnson, who led the Department of Homeland Security during President Obama's second term, said 33 states approached the department to seek cybersecurity assistance ahead of the November vote, along with officials from cities and counties.

"I know of no evidence that last year ballots were altered or votes were suppressed through a cyberattack, but last year's experience exposed certain cyber vulnerabilities in our election infrastructure," Johnson told the panel.

Johnson said the department observed "scanning and probing" of election systems, including voter registration rolls, and he feared registrations would be edited.

Last week the Department of Homeland Security notified 21 states that Russians targeted their election systems. The department did not release a list of states, and officials in at least two states notified -- California and Wisconsin -- suggested they were alerted based on bad information.

Suzanne Spaulding, former undersecretary for the department's national protection and programs directorate, testified alongside Johnson, and said confidential communication with states is essential to build trust.

"We do not share that information without their permission, and that's critically important," she said.

In January, Johnson declared election systems to be critical infrastructure, an action he said essentially means state election officials are "priority customers" if they seek assistance.

Spaulding said concern among states about federal encroachment on state autonomy could be addressed with certified third-party entities. Johnson said states might be enticed to work with federal authorities through grants.

Officials with DHS will meet with members of the National Association of Secretaries of State and other local election officials in Atlanta in two weeks to begin discussions on what the critical infrastructure designation will mean going forward.