Kanai's smooth-rigidity conjecture for frame flows

Let $(M,g)$ be a negatively curved Riemannian manifold with frame flow $\Phi_t$ on its frame bundle $FM$, and let $X^{FM}$, $E_s^{FM}$, and $E_u^{FM}$ denote the generator, stable bundle, and unstable bundle of the frame flow, respectively. Define the distribution

$$\mathbb{H}=\mathbb{R}X^{FM}\oplus E_s^{FM}\oplus E_u^{FM}.$$

Kanai's conjecture. If the distribution $\mathbb{H}$ is $\mathcal{C}^1$, then $(M,g)$ is locally symmetric.

This conjecture concerns smooth rigidity of the partially hyperbolic frame flow: enhanced regularity of the horizontal distribution should force the underlying negatively curved manifold to be locally symmetric. The supplied material does not state whether the conjecture has been resolved.