Ellen Greenberg 's parents say Philadelphia officials are still trying to clean up the city's mess from a botched investigation ... which is why no one will admit Ellen was murdered.

Sandee and Josh Greenberg joined us on "TMZ Live" Wednesday to talk about a medical examiner recently reaffirming their daughter Ellen's death as a suicide ... and, they told us it's all spurred on by corruption. change.org

According to the Greenbergs, they weren't surprised at the ruling ... because it's the answer they've gotten from city officials for years ... which they see as an attempt to simply get to them to stop digging.

They also promise they've got people in place to keep up the good fight long after the two of them are gone.

As for why authorities would allegedly collude to obfusca

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