It's a process.
First you politely invite him to come down for a chat.
When he refuses, you issue a subpoena as well as one for production of documents.
If he fails to show up, gives bad testimony (which he will) or takes the fifth , you have enough justification to issue a criminal referral to the FBI
This process should play such that the criminal referral hits the FBI shortly after the Trump's =FBI nominee, Director Wray, is confirmed and on the job.
Yes but there is no need for the first part of the process, there may even be a legal problem because I don’t think a “request” creates an obligation to preserve documents. A subpoena does. Maybe I’m mistaken and they tell them to preserve records I sort of doubt it.