We've had multiple cases where we have had help from the FBI.
I work at one of the larger webhosting companies, so we have a point of contact with them to start with. We help them out with various things (CALEA, data preservation, child porn/financial fraud, etc) so they tend to assist for major problems.
We reach out to our point of contact. We have to prove that the criminal fit somewhere in http://www.justice.gov/criminal/cybercrime/docs/ccmanual.pdf
They then ask for data (our proof). In all the cases we've handled, our proof has been enough to hand off to a prosecutor. Sometimes the data is enough for the prosecutor to move forward and score a conviction, sometimes they have doubts as to whether a jury can be convinced so they either let them take deferred adjudication, or they try to strike a plea.
We've always known the culprit personally when they commit the crime (name, address, etc.) so I can't speak for other peoples' investigations where these things aren't known.