Russian government hackers targeted 21 American states during the 2016 presidential elections in an attempt to sway the vote in favor of Donald Trump.
The Department of Homeland Security confirmed it had informed election officials in the targeted states. It did not disclose the identity of the states.
Wisconsin, Ohio, California and ten others said on Friday that they were victim of the Russian hackers.
Moscow has denied election meddling and President Trump has denied any collusion with Russia.
Being targeted does not mean that sensitive voter data was manipulated or results were changed. A hacker targeting a system without getting inside is similar to a burglar circling a house checking for unlocked doors and windows.
Homeland Security officials have said that in most of the 21 states only preliminary activity was observed from hackers and a small number of networks were compromised. Some states had complained in June they had no idea if Russians had attempted to infiltrate their systems.
The targets included voter registration systems but not vote tallying software.
"It is completely unacceptable that it has taken DHS over a year to inform our office of Russian scanning of our systems, despite our repeated requests for information," California Secretary of State Alex Padilla, a Democrat, said in a statement.
"The practice of withholding critical information from elections officials is a detriment to the security of our elections and our democracy."
Alabama, Alaska, Colorado, Connecticut, Florida, Minnesota, Texas and Washington state also confirmed they were targeted by Russian hackers but said they were not successful. Arizona and Illinois confirmed last year that they were targets.
The Associated Press confirmed Iowa, Maryland, North Dakota, Pennsylvania, Delaware, Oregon, Oklahoma and Virginia were also targets, bringing the total states identified to 21. Those states did not immediately return messages seeking comment late Friday.
“There remains no evidence that the Russians altered one vote or changed one registration,” said Judd Choate, president of the US National Association of State Election Directors.
Wisconsin Elections Commission Administrator Michael Haas said Homeland Security told the states that “Russian government cyber actors” targeted state voter registration systems.
Homeland Security spokesman Scott McConnell said in a statement the government believes “officials should be kept informed about cybersecurity risks to election infrastructure” but also wants to protect “the integrity of investigations and the confidentiality of system owners.”
US intelligence agencies have concluded the Kremlin orchestrated an operation that included hacking and online propaganda intended to help Trump win, Reuters reported in August.
Washington state’s top election official, Kim Wyman, said the state learned in 2016 of attempted intrusions from Russian internet addresses and immediately alerted the Federal Bureau of Investigation.
The list of targets includes battleground states like Wisconsin, Ohio and Iowa, but other key states like Michigan said Friday they were told they were not targeted. It also included states that were not seriously contested like California and Texas.
Wisconsin was one of a handful of battleground Midwestern states that helped Trump win the presidency over Democratic rival Hillary Clinton. Trump carried the state by 22,748 votes, or about 0.8 of a percentage point. Many of the other states were not seriously in contention in the 2016 race.
Several congressional committees are investigating and special counsel Robert Mueller is leading a separate probe into the Russia matter, including whether Moscow colluded with the Trump campaign.
Earlier this year, a leaked National Security Agency report detailed that hackers obtained information from a company that provided software to manage voter registrations in eight states. The May report said hackers sent phishing emails to 122 local election officials just before the November 2016 election in an attempt to break into their systems.
The latest disclosure to the states comes as a special counsel investigates whether there was any coordination during the 2016 presidential campaign between Russia and associates of Donald Trump.
Trump, a Republican who defeated Democrat Hillary Clinton in the presidential election, has called the Russia story a hoax. He says Russian President Vladimir Putin "vehemently denied" the conclusions of numerous American intelligence agencies.