Last I heard, the slingshots were in the car. This may have been revised though.
Cops have said there were never any slingshots at all, anywhere, not on the brothers or the dead boy, not in the car, not even anywhere in the zoo.
(I find it hard to believe they have searched every inch of ground where the guys might have thrown a slingshot away while on the run, or during that missing 20-30 minutes when no one knows where the victims were and the tiger was loose, but if the cops say “no slingshots,” I gotta go with that.)