As Nick Reiner prepares to make his first court appearance … TMZ has learned Rob and Michele had been dead for some time before their daughter made the grisly discovery Sunday.

Law enforcement sources tell TMZ … When police arrived on the scene Sunday afternoon around 3:40 PM, rigor mortis appeared to have set in Rob and Michele’s bodies — meaning they had been deceased for hours before their daughter Romy arrived.

Romy arrived at the house and found the bodies … then called police, before calling longtime family friend Billy Crystal.

Police interviewed Romy at the scene, who by then was joined by a female friend … and we hear she told officers her brother Nick was “dangerous.” We’re told she also said that Nick was living at the house — but he was not there when she arrived, insinuating he fled the scene.

Sources tell us … the scene was something out of a nightmare — with the bodies butchered … the devastation was described as the actions of a “mad man.”