  Gamma rays have been observed from two blazars at TeV energies. One of these,
Markarian 421, has been observed also at GeV energies and has roughly equal
luminosity per decade at GeV and TeV energies. Photon-photon pair production on
the infrared background radiation is expected to prevent observation above
about 1 TeV. However, the infrared background is not well known and it may be
possible to observe the nearest blazars up to energies somewhat below about 100
TeV where absorption on the cosmic microwave background will give a sharp
cut-off.
  Blazars are commonly believed to correspond to low power radio galaxies, seen
down along a relativistic jet; as such they are all expected to have the
nuclear activity encircled by a dusty molecular torus, which subtends an angle
of 90 degrees or more in width as seen from the central source. Photon-photon
pair production can also take place on the infrared radiation produced at the
AGN by this molecular torus and surrounding outer disk. We calculate the
optical depth for escaping gamma-rays produced near the central black hole and
at various points along the jet axis for the case of blazars where the
radiation is observed in a direction closely aligned with the jet.
  We find that the TeV emission site must be well above the top of the torus.
For example, if the torus has an inner radius of 0.1 pc and an outer radius of
0.2 pc, then the emission site in Mrk 421 would have be at least 0.25 pc above
the upper surface of the torus, and if Mrk 421 is observed above 50 TeV in the
future, the emission site would have to be at least 0.5 pc above the upper
surface. This has important implications for models of gamma-ray emission in
active galactic nuclei.
