U.S. and private security experts are trying to expel unidentified hackers from the unclassified portion of the U.S. State Department's email system, two officials familiar with the investigation said Thursday.
The problem persists three months after the hackers were first discovered because the intruders' techniques keep shifting, said the officials, who asked for anonymity because the inquiry is classified even though no classified material appears to have been obtained.
The attacks are worrisome because they could enable the hackers to generate false emails, delete real ones or seek a way into classified communications systems, the officials said.