A fascinating Wall Street Journal piece adds another piece to the puzzle on the Russian hacking story:

A third person familiar with the investigation said RNC staff members didn’t realize they had been the target of spies until June, after Democratic committee leaders revealed that hackers had successfully gained a foothold inside their networks. Once inside, they reportedly were able to access a trove of DNC opposition research on Mr. Trump, then a candidate.
RNC officials, concerned they too might have been compromised, called a private computer security firm, which in turn called the FBI and obtained information about what kinds of malicious emails to look for, the person said. Upon inspection, the RNC found that its electronic filters had blocked emails sent to a former employee matching the description they’d been warned about.
The apparently successful blocking of a Russian espionage operation offers one possible explanation why the GOP’s main political organization didn’t suffer the same fate as its Democratic counterpart—a deluge of leaked emails revealing private correspondence and internal strategy.