Conan O'Brien Stopped Guests From Calling 911 on Nick Reiner During Explosive Fight With Dad Rob at Holiday Party — Hours Before Bloodbath

The reported fight between Rob Reiner and his deeply troubled son Nick at Conan O'Brien's Christmas party led at least one guest to desperately beg for cops to be called, RadarOnline.com can reveal.

The former talk show host reportedly shot down the idea, saying he didn't want police at his home to cause a scene among the holiday festivities. Hours later, Nick allegedly brutally butchered Rob and his wife, Michele, in their beds at their Brentwood, California, mansion.

"They got into an argument, the father and son. It got so bad and loud that someone wanted to call the police to report it," an insider claimed to the Daily Mail on Friday, December 19.

"But Conan stepped in and said, 'It's my house, my party, I'm not calling the police.' He talked them out of calling the police," the source insisted.

"When the s--- was hitting the fan, somebody said we need to call the police. The conversation was about getting this kid put into a mental health hold," a second source told the outlet.

"Conan O'Brien stepped in and said, 'No, I don't want the police at my house.'"

"It makes sense; who wants the cops showing up, creating a scene?" the insider said about O'Brien's wish for no police. However, they noted, "But the argument between Rob and Nick was pretty intense."

Rob and Michele brought Nick to the star-studded bash out of fear of leaving him home alone after his mental health had reportedly been spiraling.

According to sources, Nick "freaked out" party guests with his disturbing and antisocial conduct, including asking the many A-listers present if they were "famous" while having an intense interaction with comedian-actor Bill Hader.

That led the When Harry Met Sally director to pull his son aside to discuss his "asocial and unsettling behavior," which escalated into such a loud disagreement that the room fell quiet as other guests could hear the commotion.

Rob and Michele reportedly apologized to O'Brien for the disturbance involving Nick and left the party to go home, although it is unclear where their son went.

He was caught on a surveillance camera walking past a gas station about two miles from his family's home shortly after midnight. A "tweaked out" Nick later checked into the Pierside Hotel in Santa Monica around 4 a.m., leaving behind a shower "full of blood" and b----- bedsheets, which were discovered by housekeeping later that morning.

However, while a family friend was unable to confirm or deny the alleged holiday party fight, he believed the situation was "misinterpreted" and being "overblown." The pal close to the family was also said to "take offense" to claims the argument led to the couple leaving the party early, which he claimed was not very likely, as the filmmaker had "grown used to" Nick's behavior over the years.

The lifeless bodies of Rob and Michele were found the following day by their daughter, Romy, around 3:30 p.m. Nick was arrested about 20 miles away at a convenience store in LA's Exposition Park area five hours later.

The first insider hoped O'Brien wasn't beating himself up for not calling the police and possibly having the situation de-escalated or having Nick taken into custody on some form of mental health hold.

"Only the murderer is responsible," said the source. "But it must be gut-wrenching for Conan to learn that this is how things turned out. I'm sure that night is replaying for him over and over. It's just so awful and sad."

Reports have since surfaced that Nick, who had a long history of drug abuse and mental illness, had been diagnosed with schizophrenia before allegedly killing his parents.

One insider claimed, "Nick was out of his head," after his medication had recently been changed.