How could that have happened?
“Hacking” of public email systems is not difficult, and I would go so far to say that Google probably gave backdoor access to government agencies. Bottom line, don’t store anything critical on public infrastructure such as Google, Amazon, Yahoo, etc.
If you need to save critical documents, either print them and file them in triplicate or have at least 3 soft copies of them (i.e. CD/DVD, thumb drive, and computer hard drive).
Inside TAO: Documents Reveal Top NSA Hacking Unit
The NSA's 50-Page Catalog Of Back Door Penetration Techniques Revealed
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