According to a joint FBI and Homeland Security document obtained by the NY Times (and confirmed by security specialists responding to the attacks), hackers have been breaching nuclear and other energy facilities and manufacturing plants since May.
The report carried an amber warning, the second highest threat-level.
The hackers posed as job applicants and sent resume attachments (MS Word docs) to agencies that operate nuclear and other facilities, and when the HR department opened the attachments, their computers were infected, and the virus traveled from that computer to others connected in the agency's internal network.
Not known or not detailed in the report: What the purpose was (whether to steal secrets, cause problems, or other); what countries the hackers were based in; and whether the hackers were able to jump from their victims computers to control systems of facilities. There is no indication of evidence for anything to this effect, according to the Times.
The report used the term "advanced persistent threat actor" for the hackers, a term often meaning "backed by a government."