This story is part of, CNET's coverage of the voting in November and its aftermath.
US officials have already warned about a rampant disinformation campaign from China, Russia and Iran to influence the 2020 presidential election, but an alert on Tuesday night put a warning out for what happens after Election Day.
In a joint statement from the Federal Bureau of Investigation and the Cybersecurity and Infrastructure Security Agency, intelligence officials said that foreign actors and cybercriminals are likely to spread disinformation on the results of the election contest between President Donald Trump and Democratic nominee Joe Biden.
Disinformation doesn't end when the polls close, and election security officials have long raised concerns about efforts to challenge the results after voting ends. But the COVID-19 pandemic and the Trump administration's remarks about mail-in voting have made it more difficult for election officials to tame disinformation online.
While Americans are used to getting results by the end of Election Day, the coronavirus outbreak has caused a surge in mail-in voting requests. The US Postal Service warned in August that some ballots may not arrive on time. That means that the presidential election's results most likely won't arrive immediately -- leaving plenty of time for disinformation to ferment online.
"The increased use of mail-in ballots due to COVID-19 protocols could leave officials with incomplete results on election night," the FBI and CISA said. "Foreign actors and cybercriminals could exploit the time required to certify and announce elections' results by disseminating disinformation that includes reports of voter suppression, cyberattacks targeting election infrastructure, voter or ballot fraud, and other problems intended to convince the public of the elections' illegitimacy."
These efforts could include creating fake websites to show unofficial "winners" of the election, hacking existing poll-tracking websites to alter the results, or spreading disinformation on social media to challenge the outcome.
Election security officials haven't found any evidence of hacked voting machines or election infrastructure like voter registration databases, or any coordinated efforts to commit mail-in voting fraud, but have seen disinformation campaigns targeting US politics.
It's much easier to have people believe their votes have been hacked than to actually do it, and falsehoods can spread quickly on social media. At a House Homeland Security hearing on Sept. 17, FBI director Chris Wray warned Americans against getting election information on social media, calling it a "mistake."
A Reuters report on Wednesday found that state and local officials struggled to stop disinformation's spread in a series of election security exercises.
Facebook is already prepping for post-Election Day disinformation efforts. Nick Clegg, the social network's head of global affairs, told the Financial Times on Tuesday that the social network has "break-glass options" to restrict posts if the election results lead to violence and chaos.
Twitter also announced new policies around misleading information about the election, including unverified reports about the results and the voting process.
The joint statement from the FBI and CISA recommends that people get their election information from their state and local officials, and also verify the information before sharing any content on social media.