Former Secretary of Homeland Security Jeh Johnson told lawmakers on Thursday that he has seen no evidence to indicate that any ballots were altered or any votes were suppressed through a cyber attack during the 2016 presidential election.

Johnson, who headed the Department of Homeland Security during the Obama administration, testified before an election security task force assembled by congressional Democrats. He told the panel that he is concerned about cyber security issues during elections but is unaware of any evidence showing cyber intrusions that affected the 2016 vote.

“I know of no evidence that last year ballots were altered or votes were suppressed through a cyber attack, but last year’s experience exposed certain cyber vulnerabilities in our election infrastructure,” Johnson said. “It was a wake up call.”

“The integrity of our election outcomes on a national level dances on the head of a pin,” Johnson said earlier in his testimony. “If writers of the TV series ‘House of Cards’ can figure that out, then a lot of other people can do the same.”

Johnson said the question now is what should be done in Washington, D.C. and at the state and local levels to patch up cyber vulnerabilities.

“My hope is that this task force and the other committees of Congress looking into this question find answers about what we do,” he said.

The task force’s forum on Thursday came one week after the Department of Homeland Security notified 21 states that Russian hackers attempted unsuccessfully to interfere with their voting systems in the 2016 election.