The U.S. Nuclear Regulatory Commission was "successfully hacked" three times in recent years in attacks involving tainted emails, according to an internal investigation on cyberattacks at the agency, Nextgov.com reported on Tuesday.

At least two of the attacks originated overseas, according to the report obtained by Nextgov, a rare public report with details of a cyberattack on the energy sector.

The publication said it obtained a copy of a report by the NRC's Office of the Inspector General, which reviewed 17 suspected breaches from 2010 to 2013.