As usual the FAA is lacking in oversight and chasing after the facts after the company has gone out of business.
Take a good look at the letter.
Are you subject of an investigation?
Are you suspected of breaking regulations?
Are you to travel at your own expenses for what exactly?
What are the consequences if you can’t make yourself available?They can do a phone conference call with your attorney or forward their questions to a FSDO local to you.
I would certainly get the opinion of an attorney if not outright bring one with you.
Anytjing you say can and will be used against you in a court of law.
You don’t get invited for coffee and cookies because they’ve heard you’re a great guy.
Read the letter again, get council and do not be naive when you deal with the FAA.