A knotted minimal tree

This paper contains a construction of a finite set X in the boundary of the unit 3-ball in R^3 whose minimal tree is knotted. The example answers Problem 5.17 in ‘‘Problems in Low-dimensional Topology’’ by Rob Kirby posed by Michael Freedman: ‘‘Given a finite set of points X in the boundary of B^3, let T be a tree in B^3 of minimal length containing X. Is T unknotted, that is, is there a PL imbedded 2-ball in B^3 containing T?’’