Nonhyperbolic Coxeter groups with Menger curve boundary (with erratum)

A generic finite presentation defines a word hyperbolic group whose boundary is homeomorphic to the Menger curve. In this article, we produce the first known examples of non-hyperbolic $CAT(0)$ groups whose visual boundary is homeomorphic to the Menger curve. The examples in question are the Coxeter groups whose nerve is a complete graph on $n$ vertices for $n\ge 5$. The construction depends on a slight extension of Sierpi'nski’s theorem on embedding $1$–dimensional planar compacta into the Sierpi'nski carpet. See the appendix for a brief erratum.