Kash Patel, President-elect Trump's pick to serve as the next FBI director, was the target of an Iran-backed cyberattack, according to senior law enforcement officials.

To understand the full scope of the damage Mr. Patel could inflict, you have to understand how unique, powerful and dangerous the F.B.I. can be — and why a Patel directorship would likely corrupt and bend the institution for decades, even if he only served a few years.