"We saw a targeting of 21 states and an exceptionally small number of them were actually successfully penetrated," Jeanette Manfra, the head of cyber-security at the US Department of Homeland Security (DHS), told NBC News on Wednesday.
Manfra, whose role is to protect American elections from hacking, did not specify which states had been successfully penetrated, saying she could not publicly disclose classified information.
"We were able to determine that the scanning and probing of voter registration databases was coming from the Russian government," she said.
However, there is still no evidence that any of the voter registration rolls were altered in any fashion, according to US officials.
In September, the US government notified election officials in 21 states that hackers carried out cyber attacks on their computer systems before the presidential election, although the systems were not breached in most cases.
Jeh Johnson, who was DHS secretary during the election, said, "2016 was a wake-up call and now it's incumbent upon states and the Feds to do something about it before our democracy is attacked again."
In January 2017, just weeks before leaving his post, Johnson designated US election systems as "critical infrastructure," a classification that applies to entities like the power grid that could be attacked. The move provides federal assistance for state and local governments to keep their election systems safe from cyber attacks.
But Johnson told NBC News he is now worried that since the 2016 election a lot of states have done little to nothing "to actually harden their cybersecurity."
Manfra said she didn't agree with Johnson's assessment. "I would say they have all taken it seriously."
Special Counsel Robert Mueller is leading the US Justice Department’s investigation into alleged collusion between President Donald Trump’s election campaign and Russians.
Republican lawmakers in Congress say the Justice Department actively sought to undermine Trump and help the election campaign of former Democratic presidential candidate Hillary Clinton.
US intelligence agencies claim Russia-linked hackers provided WikiLeaks with damaging information -- in the form of thousands of hacked emails -- about Clinton to skew the 2016 presidential election in favor of Trump.
Trump has repeatedly denied allegations that his campaign colluded with Russians and has condemned the investigations. Russian President Vladimir Putin has also denied the allegations.