The investigation into the Dogan Media Group, which owns the independent Hurriyet daily and CNN Turk channel among other outlets, was triggered by its contrasting treatment of images of Turkish soldiers and members of the outlawed Kurdistan Workers' Party (PKK), Anatolia said.
While the images of Turkish soldiers killed in August were "uncensored", meaning their features were distinguishable, those of PKK militants killed in another battle were blurred, the report added.
It comes nearly two months into the resumption of fighting between the Turkish military and PKK, after the collapse in July of a two-year-old ceasefire.
The army is pressing a relentless offensive against PKK strongholds in southeast Turkey and in northern Iraq, to which the PKK has responded with a string of deadly attacks on the security forces.
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The case against Dogan is set to add to concern over deteriorating press freedoms in Turkey under Erdogan, with a number of journalists facing legal proceedings for allegedly insulting top officials.
Turkish police also searched the Ankara-based offices of a media group critical of Erdogan earlier this month, and two British reporters were briefly jailed on terrorism charges.
The offices of Hurriyet newspaper have been attacked twice by pro-government demonstrators for allegedly misquoting Erdogan.