This NYT story by ERIC LIPTON, DAVID E. SANGER and SCOTT SHANE has the most comprehensive account that I've yet seen of how Putin intervened in the US election. As usual with hacks, incompetence of the hacked was a major factor.
A brief excerpt:
WASHINGTON — When Special Agent Adrian Hawkins of the Federal Bureau of Investigation called the Democratic National Committee in September 2015 to pass along some troubling news about its computer network, he was transferred, naturally, to the help desk.
His message was brief, if alarming. At least one computer system belonging to the D.N.C. had been compromised by hackers federal investigators had named “the Dukes,” a cyberespionage team linked to the Russian government.
The F.B.I. knew it well: The bureau had spent the last few years trying to kick the Dukes out of the unclassified email systems of the White House, the State Department and even the Joint Chiefs of Staff, one of the government’s best-protected networks.
Yared Tamene, the tech-support contractor at the D.N.C. who fielded the call, was no expert in cyberattacks. His first moves were to check Google for “the Dukes” and conduct a cursory search of the D.N.C. computer system logs to look for hints of such a cyberintrusion. By his own account, he did not look too hard even after Special Agent Hawkins called back repeatedly over the next several weeks — in part because he wasn’t certain the caller was a real F.B.I. agent and not an impostor.
And the FBI guy never got off his butt to show up in person half a mile away at DNC headquarters. We've seen this movie before, way to many times.
There is much, much more in the story, including the details of the phishing attack that led to the penetration.