  Let (M,F) be a closed manifold with a Riemannian foliation. We show that the
secondary characteristic classes of the Molino's commuting sheaf of (M,F)
vanish if (M,F) is developable and the fundamental group of M is of polynomial
growth. By theorems of \'{A}lvarez L\'{o}pez, our result implies that (M,F) is
minimizable under the same conditions. As a corollary, we show that (M,F) is
minimizable if F is of codimension 2 and the fundamental group of M is of
polynomial growth.
