In a series of tweets December 11th, Nate Silver made the case that the Comey letter was decisive.

Comey had a large, measurable impact on the race. Harder to say with Russia/Wikileaks because it was drip-drip-drip. https://t.co/LgJkfYpZCkhttps://t.co/9FYMNz763b

— Nate Silver (@NateSilver538) Dec. 11, 2016

There’s more evidence, too: Late-deciding voters broke strongly against Clinton in swing states, enough to cost her MI/WI/PA. pic.twitter.com/8r801ahDQO

— Nate Silver (@NateSilver538) Dec. 11, 2016

I’ll put it like this: Clinton would almost certainly be President-elect if the election had been held on Oct. 27 (day before Comey letter).

— Nate Silver (@NateSilver538) Dec. 11, 2016

Clinton’s chances of victory according to FiveThirtyEight dropped substantially in states after Comey’s announcement.

State                                   10/28    11/3      Drop
Florida                               65.8      49.6     16.2
North Carolina                64.4      49.5     14.9
Arizona                              49.4      28.9     20.5
Iowa                                    50.9      28.3     22.6
Ohio                                    49.9      32.7     17.2
Michigan                           86.5      78.3     8.2
Pennsylvania                   85.9      75.9     10
Wisconsin                         85.3      78.7     6.6
Average drop                                               14.5
United States                   81.5       66.2     15.3