At least 1 US nuclear plant's computer system was hacked There was no evidence that sensitive or operational systems were breached.

 -- Federal authorities were investigating a breach into computer systems of at least one U.S. nuclear power plant, sources familiar with the matter told ABC News.

There was no evidence that any particularly sensitive or operational systems were breached. Authorities believe only a less sensitive, business-associated side was compromised in at least one breach detected over recent months.

The breach was first reported by E&E News, which covers energy and environment business and policy issues.

E&E noted the hack did not garner the attention of the public safety alert systems at the Nuclear Regulatory Commission or the International Atomic Energy Agency, which could be further evidence of a low risk level associated with the breach.

One U.S. official called it an "ongoing matter" that is being investigated. No public authorities have issued word on who may be responsible, but agencies are looking at the possibility that another country may be behind the hack.

It was unclear if the case is related in any way to other known cyberattacks.