  In AdS/CFT duality, it is often argued that information behind the event
horizon is encoded even in boundary correlators. However, its implication is
not fully understood. We study a simple model which can be analyzed explicitly.
The model is a two-dimensional scalar field propagating on the s-wave sector of
the BTZ black hole formed by the gravitational collapse of a null dust. Inside
the event horizon, we placed an artificial timelike singularity where
one-parameter family of boundary conditions is permitted. We compute two-point
correlators with two operators inserted on the boundary to see if the parameter
is reflected in the correlators. In a typical case, we give an explicit form of
the boundary correlators of an initial vacuum state and show that the parameter
can be read off from them. This does not immediately imply that the asymptotic
observer can extract the information of the singularity since one cannot
control the initial state in general. Thus, we also study whether the parameter
can be read off from the correlators for a class of initial states.
