Rob Reiner and Michele were dead for hours before the gruesome discovery

If Nick Reiner is preparing for his first court appearance… TMZ has learned Rob And Michael had been dead for some time before their daughter made the grisly discovery on Sunday.

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

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

Police interviewed Romy at the scene, who was by then accompanied by a friend… and we’re told she told officers her brother Nick was “dangerous.” We’re told she also said Nick lived at the house, but he wasn’t there when she arrived, insinuating he fled the scene.

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