Let $G$ be a connected semisimple real algebraic group. Assume that $G(\bb
R)$ has no compact factors and let $\Gamma$ be a torsion-free uniform lattice
subgroup of $G(\bb R)$. Then $\Gamma$ contains a malnormal abelian subgroup
$A$. This implies that the $\tto$ factor $\vn(\Gamma)$ contains a masa $\fk A$
with Puk\'anszky invariant $\{\infty\}$.