The FBI did not tell the Democratic National Committee last fall that officials suspected it had been the target of a cyberattack, three people with knowledge of the discussions told Reuters.

And in months of follow-up conversations about the DNC's network security, the FBI did not warn party officials that the attack was being investigated as Russian government-backed espionage, the sources said.

The lack of full disclosure by the FBI prevented DNC staffers from taking steps that could have reduced the number of confidential emails and documents that were stolen, one of the sources said.