I find Catherine Herridge to be a pretty credible source, but I don't find Napolitano to be as credible.
The issue here isn't just the content on her email servers (including the top 22 top secret ones, whose content still cannot be discussed today).
The issue is also the possibility of obstruction of justice, as well as possible violation of federal corruption laws. There is a new report from the Associated Press that Hillary received substantial $$$ for paid speeches from companies that had interest in government business. The AP found that almost all of the 82 organizations that paid for or sponsored Clinton’s speeches have actively sought to sway the government (lobbying, bidding, contacting the Statement Department during her tenure).
The DNC is extremely nervous - not that the FBI will announce a decision before the convention, but that they'll announce one AFTER.
As for Trump, there might very well be a "there" there. He is being accused of fraud, and that goes far beyond civil action.
