According to the letter, which is based on testimony from James Rybicki, Comey's Chief of Staff, and Trisha Anderson, the Principal Deputy General Counsel of National Security and Cyberlaw, Comey began drafting a statement to announce the conclusion of the Hillary investigation in April or May 2016, well before he had interviewed up to 17 key witnesses. 

Meanwhile, as if that weren't bad enough, the Comey statement was also drafted before immunity deals were struck with Cheryl Mills and Heather Samuelson who seemingly ran point, along with Platte River Networks, to destroy Hillary's emails after a Congressional subpoena had been issued mandating their preservation.

Here is a summary from the Grassley/Graham letter: