Was the letter send by certified mail?
Are they asking you to voluntarily answer questions?
As you can tell I’d love to see this letter.
If they’ve made form errors your lawyer will have some fun with them.
Again, do not underestimate the FAA.
If you flew as PIC you may become victim to something your Chief Pilot omitted such as required checks or orals or written tests under Part 135.
Seriously, treat this like you’ve had a call from the Homicide division asking if you can come “by the station to answer some questions”.
Again, do NOT underestimate the FAA.
They’re a paper tiger in the face of large companies or politics but won’t hesitate to chew up and spit out a single pilot and potentially kill a career.
If you find yourself under ‘investigation’ you’re practically radioactive as far as employment is concerned and it may take 12-18 months for your name to be cleared.
Ask me how I know.....