The private investigator hired to probe Celeste Rivas‘ death case has made a harrowing revelation — he thinks there was a “sadistic” plan made to “get rid” of the teen.

Check out Steve Fischer‘s interview on “Law&Crime Sidebar with Jesse Weber” — he says the purported “scary” plan didn’t have to do with how she died, but how her body was to be dumped following her death.

He says he can’t spill much … but thinks the original plan went wrong at some point, and that the Tesla where her dismembered corpse was found in early September was supposed to be a holding spot.

Steve — who was hired by the owner of the $3.6 million Hollywood Hills home singer D4vd — who is linked to Celeste — was renting when her rotting body was discovered — also states he believes she had been deceased for a long time.

He thinks “things changed” around January or February of this year, and adds the last photo of the Lake Elsinore, California teen taken, which he has viewed, is from January 2 of this year.

And speaking of the Tesla, which, as you know, was registered to D4vd, Steve says there’s footage showing the Tesla being parked, and who parked it.

As for if Celeste was ever inside D4vd’s nearby residence … Steve tells host Jesse there’s no evidence anything happened to the teen there.

He also says he has no idea where the “Romantic Homicide” artist is … and doesn’t believe his client has been in touch with his team for anything more than lease contract negotiations.

There’s clearly loads of unanswered questions in this case — the Los Angeles Police Department has yet to name a suspect and and no arrests have been made — even after they combed through D4vd’s rented home a month ago.

Meanwhile, Los Angeles County District Attorney Nathan Hochman told TMZ this week the case hasn’t been turned over to his office … and had nothing to add.

D4vd has said nothing about the investigation … despite there being photo and video evidence of him spending time with Celeste over the years.