Truck Ford F-150 fuel door won't stay closed ...

I have a 2004 Ford F-150 pickup. The last time I fueled up, I noticed the fuel door will not stay closed. While driving it flops open and closed. I live in one of those weird states in which it is illegal to fill up your own vehicle, so didn't see what the attendant did (or didn't) do whilst filling it up. The door itself is stock, though the fuel cap is an aftermarket locking type (fuel theft isn't uncommon in the rural area in which I live.).

Any ideas on what could be busted and/or I could check? Could something have fallen off the cap when it was out? Thanks all!

Pics:

 

Apparently Ford makes a repair kit for this unit, so that is what I ended up using. The spring mechanism evidently broke. Anywho, I just thought I'd pass it along in case someone else is searching on the issue.

 

The kit wasn't too bad; it was $55.25 US. It replaces the plastic housing, springs, etc., but the original metal door (pictured in #2) is retained. Once out, it looked to me it was just the longish spring inside that was broken.

Agree about anything not buttoned down. This one probably didn't involve any safety or legal issues. I just don't like things rattling around like that.

 

Indeed the cost wasn't bad. I was afraid I was going to have to replace the metal part and either have it painted or find one in that color from a junkyard. Fortunately that was not the case.

 

Indeed, I've got it installed. It works fine now. It was one of those repairs I was thinking was going to be more of a pain than it really was.