At least according to an unnamed source quoted by the AP:
A summary of computer logs show that four aides to Vermont Sen. Bernie Sanders' presidential campaign accessed proprietary voter data compiled by the campaign of rival Hillary Clinton.
And the records also show that the aides conducted 25 separate searches of voter information and some saved the results.
That's according to a person familiar with the data logs and the breach. The person spoke on condition of anonymity because they were not authorized to speak publicly.
Needless to say, it’s an unnamed source and this is internecine political warfare. So, who knows. But this is starting to tilt in the direction of the Sanders campaign playing dirty. And perhaps illegally.