TV Producer Jill Blackstone -- Cops Think Sister's Death was Suicide Pact

Jill Blackstone, the "Jerry Springer" and "Sally Jessy Raphael" producer who was arrested for allegedly killing her sister, engineered a suicide pact but got cold feet at the last minute ... that's the operating theory of law enforcement.

Our law enforcement sources tell us, they believe Blackstone and her sister Wendy -- who was deaf and partially blind -- made the suicide decision and Jill executed the plan by putting a lit BBQ in their closed garage, which was supposed to fill the air with deadly carbon monoxide gas.

TMZ broke the story, Jill called a friend who called 911, and when paramedics arrived they found Wendy dead and Jill with severe carbon monoxide poisoning. They believe Jill was trying to kill herself along with Wendy but had a change of heart at the very last moment.

And cops believe the 2 dogs that died were part of the plan ... they would all pass together.

As we reported, police arrested Jill for murder but the D.A. sent the case back to the LAPD for further investigation and Jill was released.