Ellen Greenberg 's 2011 death from multiple stab wounds in the back of her head and neck was ruled a suicide once again ... but a forensic pathologist tells TMZ that's "biomechanically implausible."

Veteran death investigator Joseph Scott Morgan joined us on "TMZ Live" Tuesday, saying he was shocked when he heard the ruling ... "You could've knocked me over with a feather, as they say."

He continued, "We thought that science would prevail. None of us really saw this coming."

As you know ... Greenberg was a 27-year-old Philadelphia teacher found dead by her fiancé ... she'd been stabbed 20 times in the back of her neck and head, with a knife protruding from her chest. While her death was initially determined to be a homicide, an assistant medical examiner ruled it was suicide. cha

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