The officials, who spoke on condition of anonymity, said however, that the U.S. election system is so large, diffuse and antiquated that hackers would not be able to change the outcome of the Nov. 8 election.

But hackers could post documents, some of which might be falsified, that are designed to create public perceptions of widespread voter fraud, the officials said.

They said that they did not have specific evidence of such a plan, but state and local election authorities had been warned to be vigilant for hacking attempts.