Ex Power Ranger -- Ricardo Medina Jr. to Be Released in Murder Case, No Charges ... Yet

Ricardo Medina Jr. -- the former Power Ranger arrested for stabbing his roommate to death -- is about to walk free ... while cops investigate and prosecutors decide whether to charge him.

The L.A. County D.A.'s Office declined to charge Medina -- for now -- and instead asked Sheriff's investigators to dig for more information about the fight that ended with Medina fatally stabbing Joshua Sutter with a sword.

Prosecutors had to release Medina now because they're not charging him with a crime yet.

Law enforcement sources tell TMZ ... Medina is claiming he acted in self-defense. As we've reported ... Medina told cops Sutter forced his way into his bedroom before the stabbing. The question for investigators -- would stabbing an unarmed man constitute self-defense?