Voters should be on high alert for foreign interference into the 3 November US election, the country's counterintelligence agency chief has said.
It's a rare warning that the public should screen information, check online sources and report suspicious actions.
William Evanina, chief of the National Counterintelligence and Security Center (NCSC), said that with the election close it was "imperative" his agency share with voters some of the information about possible interference that US spies has also shared with legislators and campaigns.
"The American public has a role to play in securing the election, particularly in maintaining vigilance against foreign influence," said Evanina.
"At the most basic level, we encourage Americans to consume information with a critical eye, check out sources before reposting or spreading messages, practice good cyber hygiene and media literacy, and report suspicious election-related activity to authorities," he added in a written statement.
A January 2017 US intelligence community assessment found Russia meddled in the 2016 US election, with the goal to aid now- president Donald Trump. Trump has cast doubt on whether Moscow interfered in the vote.
Given the complexity of US vote counting and audit systems, Evanina said it would be "extraordinarily difficult for foreign adversaries to broadly disrupt or change vote tallies without detection."
However, Evanina said US spy agencies now see foreign adversaries trying to compromise US political campaigns and candidates, as well as the election infrastructure.
He said foreign nations are trying to influence voters in the US via social and traditional media, using issues such as the coronavirus pandemic and domestic protests as fodder for disinformation.
Evanina said his agency was mainly concerned with interference by China, Russia and Iran, though other unnamed countries and non-state actors "could also do harm to our electoral process."