The attack against U.S. democracy began in the summer of 2015 with a simple trick: Hackers working for Russia's civilian intelligence service sent emails with hidden malware to more than 1,000 people working for the American government and political groups.

U.S. intelligence agencies say that was the modest start of "Grizzly Steppe," their name for what they say developed into a far-reaching Russian operation to interfere with this year's presidential election.

Prodded to produce evidence by Russia, which has denied a role in hacking — and by an openly skeptical President-elect Donald Trump — the FBI and the Department of Homeland Security did so Thursday. They issued a 13-page joint analysis just as President Barack Obama imposed sanctions against Russian government organizations and individuals and expelled 35 Russian operatives.