Russian hacking of U.S. electoral systems hit nearly twice as many U.S. states as previously reported, the Bloomberg news agency said on June 13, citing people familiar with the investigation.
The Bloomberg report said Russian hackers hit 39 of the 50 U.S. states in the summer and autumn of 2016 ahead of the presidential election in November.
The report said that the hackers also tried to alter or delete voting records.
U.S. President Barack Obama's government was so concerned by the interference that it complained directly to Moscow, using a "red phone" for secure, direct communication between the governments, the report added.
An ongoing probe of Russian interference in the U.S. elections has largely focused on the hacking of Democratic Party emails but has expanded to questions about possible coordination with U.S. President Donald Trump's campaign.
Authorities had feared widespread disruption in the elections, but investigations so far have turned up no impact from hacking on voting totals or the conduct of the election itself.
Such an effort would be complicated by the decentralized election system used in the United States, in which local officials oversee voting in different ways from jurisdiction to jurisdiction.
The FBI declined to comment on the details of the report.
Former FBI director James Comey warned last week that Russian efforts to influence elections would continue and that their focus includes the undermining of confidence in the democratic system.