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.”

Rob Reiner and his wife Michele’s deaths have sent shockwaves through Hollywood … and Donnell Rawlings is opening up to TMZ about the tremendous impact Rob had on the entertainment industry, and how profound the loss of the couple is.

We caught up with the comedian at LAX Monday morning, and he told us that while it’ll be incredibly tough, he hopes Rob’s family and friends can eventually find some understanding of how this tragedy happened.

Catch the full clip — Donnell looks back on Rob’s groundbreaking career, talking about how he embraced change when others wouldn’t and used his platform to open doors for people along the way.