  Observations of structure at sub-galactic scales are crucial for probing the
properties of dark matter, which is the dominant source of gravity in the
universe. It will become increasingly important for future surveys to
distinguish between line-of-sight halos and subhalos to avoid wrong inferences
on the nature of dark matter. We reanalyze a sub-galactic structure (in lens
JVAS B1938+666) that has been previously found using the gravitational imaging
technique in galaxy-galaxy lensing systems. This structure has been assumed to
be a satellite in the halo of the main lens galaxy. We fit the redshift of the
perturber of the system as a free parameter, using the multi-plane thin-lens
approximation, and find that the redshift of the perturber is $z_\mathrm{int} =
1.42\substack{+0.10 \\ -0.15}$ (with a main lens redshift of $z=0.881$). Our
analysis indicates that this structure is more massive than the previous result
by an order of magnitude. This constitutes the first dark perturber shown to be
a line-of-sight halo with a gravitational lensing method.
