The UN Security Council has already made clear that those responsible need to be brought to justice. I think keeping things well within the UN would be a good idea for the US right now, because I’m pretty sure this is something the members will agree on easily. That means: more agreement in the UNSC and also the US playing by the rules which makes everyone happy.
The UN Secretary-General Ban Ki-moon has already stated that Libyan authorities need to release statements about bringing those responsible to justice. Because the country is obviously a mess it might be necessary to get a resolution from the UNSC to assist the Libyans. Then the US can ask to have them extradited.
Anyway, that seems like a better idea to me than the US going in there for some secret missions and pissing off all manner of people. Also because the entire international community needs to make clear that killing ambassadors will always be detrimental to your cause.
Just what I would think; I’m no expert.