A U.S. cybersecurity official said Wednesday that Russia "successfully penetrated" the voter rolls in a small number of states in 2016.

Jeanette Manfra, the head of cybersecurity at the Department of Homeland Security (DHS), told NBC News that Russia targeted 21 states and “an exceptionally small number of them were actually successfully penetrated.”

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DHS previously notified the 21 states that Russia had attempted to hack their elections systems before the 2016 election.

It was Manfra who first revealed to the Senate Intelligence Committee last June that the states had their systems targeted by Russian hackers ahead of the election.

It was previously known that voter registration databases in Arizona and Illinois were breached by hackers. Alabama, California, Colorado, Wisconsin and Florida are among the other states that have confirmed they were targeted.

Officials told NBC there is no evidence any of the voter rolls were altered in any way.

Homeland Security formally notified election officials in the states that were targeted. Officials said then that most of the targeting amounted to mere preparations for hacking, such as probing for vulnerabilities.

The targeting was part of a broader effort by Moscow to meddle in the presidential election, according to the U.S. intelligence community. The systems targeted were not involved in vote tallying.

The revelations have sparked widespread fears that Russia or another foreign actor could seek to interfere in future elections using cyberattacks and other tactics.

Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE warned Tuesday that Russia is already attempting to meddle in the U.S. midterm elections this year.

But Manfra told The Hill in a recent interview that, while she is unaware of any credible targeting efforts related to the 2018 midterm elections, she remains worried about the threat.

“I will always be worried about it and it is always something that entities are going to look to influence our democratic processes,” she said. “As a country, we should be in a position to counter that.”

The department is providing vulnerability tests and other services to states looking to shore up the cybersecurity of their election systems ahead of future votes, as part of its new designation of voting systems as critical infrastructure.

Homeland Security is also working with state election officials to share information on cyber threats.

This story was updated at 5:40 p.m.