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.

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.”

Billy Crystal and his wife Janice saw the slain bodies of Rob Reiner and his wife Michele … because TMZ has learned they arrived at the scene in a flash after a family member called them about the horrific discovery.

Sources with direct knowledge tell TMZ … Rob and Michele’s daughter Romy called Crystal and his wife not long after she called police to report finding her parents brutally murdered inside their Brentwood home Sunday.

Sources with direct knowledge tell us Billy and his wife raced over to the house — and were there so quickly, one source tells us — the couple actually saw their slain friends … and wanted to say their goodbyes.

Tanya Brown — whose sister, Nicole Brown, was slain in 1994 — opened up to TMZ about the eerie similarities between Rob and Michele Reiner‘s killings and the brutal murder of her sister, which sparked the infamous O.J. Simpson murder trial.

Tanya tells TMZ … she saw the words “murder in Brentwood” posted on social media Sunday night, which understandably conjured up old, terrible memories.

As we reported … Rob and his wife were found Sunday with their throats slit inside their Brentwood, California home.

Tanya said, “The story is very triggering — beyond triggering. I’ve shed a lot of tears … it’s very personal to me.”

While no one was ever criminally convicted in Nicole and Ron’s murders, Tanya says she hopes the Reiner family gets justice and “the person responsible doesn’t get away with it.”

“But I hope the person also gets rehabilitated while incarcerated,” she said. “It makes me sad for the family, I feel for them. This didn’t need to happen.”