The intruders so thoroughly compromised the DNC's system that they also were able to read all email and chat traffic, said DNC officials and the security experts.

The intrusion into the DNC was one of several targeting American political organizations. The networks of presidential candidates Hillary Clinton and Donald Trump were also targeted by Russian spies, as were the computers of some GOP political action committees, U.S. officials said. But details on those cases were not available.

A Russian Embassy spokesman said he had no knowledge of such intrusions.