Nick Reiner Held on Suicide Watch After Arrest on Suspicion of Killing Parents

Nick Reiner is on suicide watch after being arrested on suspicion of murdering his parents, Rob and Michele Reiner ... TMZ has learned.

Law enforcement sources tell TMZ ... Nick is on suicide watch and in administrative segregation at Twin Towers Correctional Facility in L.A. ... while he's being held without bail.

As we reported ... prior to Rob and Michele's stabbing, Nick got into a "very loud argument" with his father at Conan O'Brien's Christmas party Saturday night. We're told the shouting match was loud enough for many others to hear, and that Rob and Michele then left the party. We do not know if Nick left with them.

A source told us Nick looked out of place at the party, mostly keeping to himself.

Our family sources told us … Michele had been anguishing to friends over the last few months that she and Rob were at their wits' end over Nick's mental illness and alleged substance abuse issues, and did not know what to do with their son Nick ... saying, "We've tried everything."

As you know ... the legendary actor-director and his wife were found in their L.A.-area house Sunday with their throats slit. Nick was arrested Monday morning and booked for murder.