A top Department of Homeland Security official said Tuesday that Russian hackers likely targeted more than 21 states before the 2016 election as part of a broader effort to interfere in the vote.

Jeanette Manfra, the official, acknowledged that the department only had enough “visibility” to confirm activity targeting 21 states because of sensors in place in the state systems and information provided by the intelligence community.

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“I think we can assume that the majority of the states were probably a target,” Manfra said during a Senate Homeland Security Committee hearing in response to questioning from ranking member Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (D-Mo.).

But Manfra pushed back on McCaskill’s assertion that states where activity was not detected were likely more vulnerable to Russian hackers because they didn't have tools in place to detect breach attempts. Manfra noted that most of the activity Homeland Security analyzed involved hackers scanning for vulnerabilities, rather than trying to break into systems.

Manfra stressed that only a small number of state systems were actually breached.

“I think the American people have been misled here,” McCaskill fumed. “It’s likely that all 50 states were likely affected.”

Homeland Security officials said last year that Russian hackers targeted election-related digital systems in 21 states before the 2016 presidential election. Federal officials maintain that hackers did not target systems involved in vote-tallying, and that there is no evidence any vote tallies were changed.

While federal officials have not offered much specific information on the targeting, officials in Illinois have said that the state voter registration database was breached. Arizona also found malware on a local election official’s computer, but officials say they shut down the state's voter registration database before hackers made their way in.

After detecting the Russian activity in 2016, Homeland Security opened election infrastructure up to voluntary federal protections. Manfra said Tuesday that 17 states had requested rigorous, on-site vulnerability assessments of their systems to shore up security ahead of future elections.

Manfra told lawmakers that the agency has adopted an “aggressive posture” to prevent future election meddling.

“We cannot let it happen it again,” Manfra said.