My guess is that issuing subpoenas for loan application data of a given target is standard practice among federal prosecutors when looking to destroy or pressure someone in cases where there is little evidence of the crime that actually sparked the investigation. They fire a shotgun and watch to see which pellets hit. Income inflation is one of the most common types of fraud perpetrated by consumers - even innocently overstating one’s income by rounding up ($100k stated vs $95k actual) on an online credit card application can technically lead to felony charges like this. So it’s at least possible that they discovered the crime at issue here through subpoenaed loan documents, and the emails only provided additional evidence. Once you’re on the government’s shit list, it’s only a matter of time before they find something that you may not even know you did that is technically a crime.