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.”

Now — following pressure from Greenberg’s family to revisit the case — Philadelphia’s Chief Medical Examiner Dr. Lindsay Simon has doubled down … completely baffling Morgan.

“It’s biomechanically implausible,” he tells us before adding, “There’s no indication she was in some mental state where she would react in this frenzied manner.”

Morgan — who’s worked for law enforcement in New Orleans and Atlanta — also says at some point a neuropathologist observed the samples of Greenberg’s brain stem, concluding that in that area — where the knife had passed through — there was no hemorrhage.

He says, “That leads us, scientifically, to conclude that this is potentially a post-mortem injury.”

According to Morgan, officials could stand to be more transparent regarding the case … “There’s never been a presser where these people have been asked questions specifically about this. Explain your rationale for this, because we want to understand.”

TMZ reached out to the Medical Examiner for an explanation on how they think Ellen could’ve done it herself … so far, no word back.