The photo first appeared on Reddit moments ago ... posted by someone who claims the photo was obtained by a "friend" who works downtown.
The picture shows the lifeless corpse of 26-year-old Tamerlan laying on a table.
The body has multiple bullet wounds ... as well as a gigantic gash along his chest rib cage area.
The body is covered in blood ... especially in the facial region.
In the death photo, Tamerlan's mouth and eyes are open wide.
Law enforcement sources tell TMZ ... the photo is authentic. One source tells us officials are investigating to find out who leaked the picture.
Tamerlan was killed in a shootout with police in Watertown, Massachusetts early this morning. His 19-year-old brother Dzhokhar is on the run.
Both are suspects in the Boston Marathon bombing that killed 3 people and injured nearly 200 more.