It would have been a great hoax if they'd planned it, but at the time, I think it was thoroughly investigated. Mylar balloon, probably.

This scientist speculates those things could conceivably travel for 1000 miles. https://www.quora.com/When-a-child-lets-go-an-average-sized-helium-balloon-and-it-flies-off-how-far-will-it-travel-before-descending