Russian hackers targeted election systems belonging to 21 US states in last year's presidential election, a US official told Congress.
Jeanatte Manfra, the acting deputy undersecretary of cybersecurity at the Department of Homeland Security, declined to identify which states had been targeted, citing confidentiality agreements.
Meanwhile, former US Homeland Security secretary Jeh Johnson told a congressional panel that there was a delay between the time the FBI first made contact with the Democratic National Committee about Russia hacking its servers and the time he was notified at the Department of Homeland Security.
Mr Johnson, who served under President Barack Obama, was testifying about alleged Russian meddling in the 2016 US presidential election.
The Department of Homeland Security issued warnings about hacking into voter registration databases, but Mr Johnson said the notices did not get the attention he would have liked.
He blamed the emergence of a 2005 tape, in which Donald Trump brags about sexually assaulting women, for distracting the US public at the time of the warnings.
Asked why the Obama administration did not do more to warn the public about the hacking, Mr Johnson said, "We were very concerned that we would not be perceived as taking sides in the election, injecting ourselves into a very heated campaign."
