Normal Subgroup Theorem for groups acting on ${\tilde{A}}_2$-buildings

Let $\mathrm{\Gamma }$ be a group acting on with finite stabilizers and finite fundamental domain on a building of type ${\tilde{A}}_2$. We prove that any non-trivial normal subgroup of $\mathrm{\Gamma }$ is of finite index in $\mathrm{\Gamma }$.