WASHINGTON (Reuters) - Cyber actors affiliated with Russia’s government conducted an “unprecedented, coordinated” campaign against the U.S. election infrastructure, a U.S. Senate committee said on Tuesday, including successfully penetrating a few voter registration databases.

The cyber attacks targeted at least 18 states, and possibly three more, the U.S. Senate Intelligence Committee said in an unclassified summary of the first installment of a report on possible Russian meddling in the 2016 election.

It said it found “ample evidence” that the Russian government sought to undermine confidence in the U.S. election infrastructure and warned of continuing vulnerabilities and the possibility of more attacks to come.

However, the panel did not find anything to show that hackers altered actual vote counts or changed any votes.

The report said that other states besides the 21 also saw suspicious or malicious behavior, which U.S. intelligence agencies have been unable to attribute to Russia.

The committee also prepared a classified report on threats to the election infrastructure.

The release marked the panel’s first public report after more than a year looking into findings that Russia sought to interfere in the campaign, including whether President Donald Trump’s associates colluded with Moscow.

Russia’s government denies seeking to interfere in the election. Trump denies collusion and has called congressional probes and an investigation by Special Counsel Robert Mueller politically motivated witch hunts.

The committee found that Russia undertook a wide variety of intelligence-related activities targeting the U.S. voting process, beginning at least as early as 2014 and continuing through Election Day 2016.

“The committee has not seen any evidence that vote tallies were manipulated or that voter registration information was deleted or modified,” the committee said in a statement.

The report said the Department of Homeland Security and Federal Bureau of Investigation alerted states to the threat during 2016, but the warnings “were limited in substance and distribution.”

As a result, it said, state officials understood there was a threat, but did not understand its scope or seriousness. It said DHS is engaging state election officials more seriously now.

The report’s recommendations included that states should continue to run elections and the U.S. government should “clearly communicate” that it will respond to attacks on election infrastructure as a hostile act, and respond accordingly.

It also said the United States should build a stronger defense, including quickly determining who is behind cyber attacks and improve communications.