Voting systems across the US were far more susceptible to Russian hacking efforts during last year's presidential election than was previously known, according to evidence uncovered during the federal government's investigation and a leaked classified intelligence report.

A wave of attacks in the summer and fall of 2016 targeted and accessed a campaign finance database and software that poll workers were supposed to use on Election Day, Bloomberg reports, citing people with knowledge of the government's investigation into the hacking.

The FBI previously confirmed that hackers breached voter registration databases in Illinois and Arizona, but the investigation has since determined that voting systems in 39 states were compromised, according to Bloomberg. The Obama administration complained about the hacking to Russian officials via rarely used diplomatic channels, including a "red phone," before ultimately sanctioning Russia in December for its involvement in the hacks.

A classified report from the National Security Agency that The Intercept published last week provides some insight into the investigation. "Russian intelligence obtained and maintained access to elements of multiple US state or local electoral boards," the NSA wrote, although "the types of systems we observed Russian actors targeting or compromising are not involved in vote tallying."

Russian president Vladimir Putin has repeatedly denied the hacking accusations, although he recently appeared to suggest that Democratic presidential candidate Hillary Clinton may have prompted private Russian hackers to attempt to compromise the US election of their own accord.

"If they are patriotically minded, they start making their contributions — which are right, from their point of view — to the fight against those who say bad things about Russia," Putin said of the hackers during a TV interview earlier this month.