Dominique Dunne was a rising star — delivering the long-lasting line in “Poltergeist” — earlier than her life was tragically taken by the hands of her then-boyfriend … and there is a lady who was not allowed to testify who may have modified the end result of his trial.

The decades-old case towards John Sweeney — the person who strangled the younger star again in 1982 — is being reexamined anew in a documentary produced by TMZ Studios … which places a contemporary mild on the trial, the aftermath …. and presents surprising new proof.

On the time, prosecutors charged Sweeney with first-degree homicide case. He was convicted of manslaughter, and he was again on the streets after solely 2 1/2 years behind bars.

The jurors you will hear from within the documentary had been shocked once we confirmed them our interview with Sweeney’s prior girlfriend, Lillian Pierce, telling us had they heard from her they might have thrown the e-book at Sweeney.

Pierce was not allowed to testify in the course of the trial … the choose felt it will be unfair to Sweeney if she defined how he had repeatedly brutalized her. Pierce has by no means spoken publicly concerning the abuse she suffered by the hands of Sweeney, till now.

Take a look at the video … what Pierce went by was horrific, and it mirrors a few of what Dominique skilled.  She says Sweeney was wired to assault his girlfriend when he acquired mad.

What’s extra, we discovered yet one more girlfriend whom Sweeney brutalized AFTER he was launched from jail.

Our doc, “Killing a Film Star: Grave Injustice” airs Monday on FOX at 9/8c.