What happens when your server is compromised and you get help from the FBI?

Often when a company announces that its servers have been compromised, the company also states that they have begun working with the FBI or a similar law enforcement agency.

What does this actually entail? I'm imagining this means anything from simply beginning contact with the FBI, to sharing server logs with them, to giving them full access to the machine for full forensics and interviewing you and your staff.

For minor intrusions, I'd imagine that there are many cases when law enforcement wouldn't get involved at all due to limited resources. On the other end of the spectrum, when millions of users' financial records are compromised I would imagine a more detailed response.

Does anyone have any experience with this? I'm curious to know anyone's stories.