  In this short note, we investigate an inverse source problem associated with
a nonlocal elliptic equation
  $\left( -\nabla \cdot \sigma \nabla \right)^s u =F$ that is given in a
bounded open set $\Omega\subset \mathbb{R}^n$, for $n\geq 3$ and $0<s<1$. We
demonstrate both $\sigma$ and $F$ can be determined uniquely by using the
exterior Dirichlet-to-Neumann (DN) map in $\Omega_e:=\mathbb{R}^n\setminus
\overline{\Omega}$. The result is intriguing in that analogous theory cannot be
true for the local case generally, that is, $s=1$. The key ingredients to prove
the uniqueness is based on the unique continuation principle for nonlocal
elliptic operators and the reduction from the nonlocal to the local via the
Stinga-Torrea extension problem.
