I saw the following question from the "Problem Section" in Schoen and Yau, page 281, problem 12:

Let $M_1, M_2$ each have negative curvature. If $\pi_1 (M_1)=\pi_1 (M_2)$, prove that $M_1$ is differeomorphic to $M_2$.

The authors then commented: " There is some progress due to Cheeger, Gromov, Farrell and Hsiang. Cheeger proved that $\pi_1(M)$ determines the second Stiefel bundle of $M$. Gromov proved $\pi_{1}(M)$ determines the unit tangent bundle of $M$. Farrell-Hsiang proved $\pi_{1}(M)$ determines $M\times \mathbb{R}^{3}$. Farrel-Hsiang have only to assume that one of the manifolds has negative curvature."

Here the paper due to Cheeger&Gromov is this one. I am unable to find any reference on Farrell&Hsiang's work as the book says it was to be published. May I ask if there is any significant progress on this problem? This is obviously some strong rigidity theorem (if turned out to be true). A quick "scan" of the papers involved in the end of Gromov's paper did not reveal anything particularly relevant.