It hasn't been that long. Teens have killed people and gotten away with it for years.
For a group of teens to pull this off, leave no evidence pointing to them behind, not say anything about it, and stay off the FBI's radar for three weeks would be extremely rare. Not to mention that they'd have to be relatively unknown to local police as any prior run-ins with local LE would immediately put them under a microscope.
It's so unlikely that it isn't really worth considering.