Acting Director Melson met investigators in secret
Dave Workman has the story. Melson was scheduled for a formal interview later, at which he'd be accompanied by Justice attorneys. Instead he met with the investigators, bringing his personal attorney, on July 4. I think the day is significant. Government attorneys would be off-duty. Government employees would be at home, reducing the chance of accidentally being seen and identified. And Melson wouldn't be on duty, so no one would wonder where he'd gone. I know from my gov't days that Justice doesn't see itself as attorney for your agency, let alone for you. It sees itself as attorneys for the entire government (and at the higher, appointed, levels, as attorneys for the Administration. They throw agencies (let alone acting agency heads) under the bus whenever required to do so.
Melson was in a tight bind. He probably read the stories about his forthcoming resignation as leaks meant to prepare him for becoming the fall guy in this. He was once a careerist with full civil service protection (to the extent the higher levels care about that), then he became SES, with lessened protection, and may even have become an appointee (that I'm not sure about) who can be fired at will. He's about to get shoved out the door. When he goes to the investigators, he's desperate, essentially defecting.
How much did he tell them? I think we'll know soon. He's apparently pointing the finger at Justice, which would mean at least an Assistant AG (political appointee), and very likely AG Holder himself, who certainly would have approved whatever the AAG was doing.
I've never heard of an agency head defecting, as it were. Closest think I can think of is John Dean, counsel to the President, meeting secretly with investigators during Watergate, and maybe "deep throat," Mark Felt, who was Associate Director of the FBI, leaking to the press. And Watergate involved quite a bit less than running thousands of guns to the most violent criminals on earth with fatal results.