Byron York unpacks the twists and turns in the Hillary e-mail scandal and coverup, here, from which he reaches this conclusion:
But even with the fragments now public — the Clinton 302, the overview report, but none of the many other witness interviews — it seems fair to conclude that staff on the Clinton team destroyed material that was under subpoena. Whether that was unwitting, or whether it was something else, is not known, at least publicly.
There's no doubt that Clinton intended to destroy her email archives after choosing which ones to send to the State Department. That's what Mills' December 2014 directive was apparently intended to do. Given that the Benghazi investigation was well under way and there had been multiple document requests and production agreements, the effect of Mills' directive, had it been carried out at the time she sent it, would have been to destroy the evidence that had not been handed over to the State Department before anyone knew to ask for it. But apparently the Platte River Networks staffer's carelessness led to the emails not being destroyed in December 2014, remaining in existence until March 2015, when their existence was publicly disclosed and another subpoena issued for them. Then they were destroyed.