Cybersecurity surrounding the U.S. electoral system has been a focus this election cycle, not least since Russian hackers have actually found their way into the Democratic Party’s network and state voter rolls, and WikiLeaks has apparently obtained and published emails allegedly belonging to Hillary Clinton campaign chair John Podesta.

Republican nominee Donald Trump has repeatedly claimed — without any evidence whatsoever — that the election might be “rigged” against him. As he cast uncertainty over whether he would accept the Nov. 8 vote’s result in Wednesday’s final televised presidential debate, U.S. intelligence and law-enforcement sources are warning that Russian hackers could undermine the American public’s trust in the electoral process by posting fake evidence of voter fraud online.

Reuters reports Thursday that these officials, speaking on conditions of anonymity, said that hackers affiliated to Russian intelligence agencies could produce fake documents purporting to show signs of electoral rigging and disseminate them online, in the hopes of creating public perception of widespread foul play in the general election.