Voting was disrupted by bomb threats and technical problems in at least four US battleground states on election day, according to officials.
The FBI said on Tuesday evening it is "aware of bomb threats to polling locations in several states, many of which appear to originate from Russian email domains", adding that they have all been deemed "non-credible" so far.
In the swing state of Pennsylvania, which carries the most Electoral College votes of all the battlegrounds, courts approved up to an hour extra voting time after bomb threats briefly stopped ballots being cast.
Ten polling stations in Georgia's Fulton County were subject to threats, with two closed and evacuated for 30 minutes on Tuesday morning.
Bomb sweeps were also carried out by police at four polling stations in DeKalb County, Georgia, with voting briefly suspended before staff were given the all-clear to return.
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Georgia's secretary of state Brad Raffensperger echoed the FBI, saying most of the threats appeared to have been sent from Russia.
"They're up to mischief, it seems. They don't want us to have a smooth, fair and accurate election, and if they can get us to fight among ourselves, they can count that as a victory," he said.
The Russian embassy in Washington denied any claims of interference, describing such allegations as "malicious slander".
"We would like to emphasise that Russia has not interfered and does not interfere in the internal affairs of other countries, including the United States," it said. "As President Vladimir Putin has repeatedly stressed, we respect the will of the American people."
In Apache County, Arizona, polls were allowed to stay open until 9pm local time - two hours after the original deadline - following bomb threats.
Adrian Fontes, Arizona's Democratic secretary of state, said four bomb hoaxes had been delivered to polling stations in Navajo County, but they did not disrupt polling there.
Pennsylvania governor Josh Shapiro sought to assure people in his state the fake bomb threats posed "no threat to the public".
Meanwhile in North Carolina, two counties suffered technical problems, according to the state's board of elections.
Gardners School precinct in Wilson County reported a printer issue that saw voting suspended between roughly 6.30am and 8am, local media said.
A computer issue was also reported at Burke County Pilot Mountain voting precinct.
The US election is expected to be decided on the results in seven so-called battleground states: North Carolina, Georgia, Pennsylvania, Michigan, Wisconsin, Nevada and Arizona.
North Carolina and Georgia originally sought permission to extend polling hours beyond their original deadlines - but Sky's partner network NBC News later confirmed both states would close on time - by 7.30pm EST (12.30am UK).
Voters were still allowed to cast their votes for a short period after that - but the polls were declared officially closed statewide.
More security threats in other battleground states
Bomb hoaxes were also reported in Michigan and Wisconsin.
Ann Jacobs, head of the Wisconsin Election Commission, confirmed two polling stations in the state capital of Madison were sent bomb threats - but neither stopped people casting their votes. She did not know of any Russian involvement.
Officials had feared disruption in North Carolina after Hurricane Helene caused devastation there last month - but authorities reported "minimal problems" aside from the minor technical glitches, with temporary precincts set up in tents in the worst-affected areas.
(c) Sky News 2024: US election: Bomb threats and printer glitches disrupt voting in battleground states