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The production site of gamma rays in blazars is closely related to theirinteraction with the photon fields surrounding the active galactic nucleus. Inthis work we discuss an indirect method that may help to unveil the presence ofambient structures in BL Lacs through the analysis of their gamma-ray spectrum.Passing through structures at different distances from the black hole, gammarays interact with the corresponding photon fields via gamma-gamma pairproduction, producing absorption features in their spectral energydistribution. An interaction of the gamma-ray photons with a putativebroad-line region may reduce the gamma-ray flux only if its production sitewere very close to the central engine. On the other hand, if jet photonsinteract with optical-UV seed photons produced by a pc-scale narrow-lineregion, the consequent gamma-gamma process may cause absorption features at afew hundreds GeV. Sources with spectra reaching TeV energies, such as HBLs andEHBLs (extreme blazars), may represent exceptional probes to investigate thistopic. In this regard, we discuss recent observations of sources which may showevidence of such absorption features in their gamma-ray spectra. Finally, wediscuss how sub-TeV absorption features in the spectra of BL Lacs may affecttheir broadband modeling, and eventually represent a powerful diagnostic toolto constrain the gamma-ray production site and the jet environment.

Absorption features in gamma-ray spectra of BL Lac objects