The Los Angeles County Medical Examiner has officially confirmed that Rob and Michele Reiner died from "multiple sharp force injuries."
Both deaths have been classified as homicides.
The Hollywood director, 78, and his photographer wife, 70, were discovered at their $13.5million Brentwood mansion on Sunday.
Their son Nick Reiner, 32, was arrested the same day.
He now faces charges of two counts of first-degree murder.
Prosecutors have added special circumstances to the charges, citing multiple murders and the use of a knife as a dangerous weapon.
If convicted, Nick could receive life imprisonment without parole - or potentially the death penalty.
Nick Reiner appeared briefly in court on Wednesday wearing an anti-suicide vest and did not enter a plea during the hearing.
His defence attorney, Alan Jackson, requested more time to prepare the case.
When the judge asked blank-faced Nick "if he agreed that his arraignment should be continued, he answered: 'Yes, your honor.'"
The arraignment has been rescheduled for January 7.
Rob Reiner and his wife Michele Singer were found dead in their home on Sunday | GETTYJackson spoke to reporters following the proceedings.
"This is a devastating tragedy that has befallen the Reiner family," Jackson said.
"There are very, very complex and serious issues that are associated with this case. These need to be thoroughly and very carefully dealt with."
Jackson urged the public not to "rush to judgment" about the case.
"We ask that throughout this process you allow the system to move forward in the way that it was designed to move forward, not with a rush to judgment," he said.
THE ROB REINER CASE - AS IT HAPPENED:
COURT SKETCH: Nick Reiner, wearing a blue protective vest, makes his first court appearance on murder charges for the killing of his parents
|REUTERS
Defence attorney Alan Jackson (right) requested more time to prepare the case
|REUTERS
Shortly after the hearing, Rob and Michele's other children released a statement.
"Words cannot even begin to describe the unimaginable pain we are experiencing every moment of the day," Romy and Jake said.
"The horrific and devastating loss of our parents, Rob and Michele Reiner, is something that no one should ever experience."
The siblings described their parents as their "best friends."
They asked for privacy and urged the public to treat speculation with "compassion and humanity."
Nick Reiner (right) has been formally charged with two counts of first degree murder after his parents' killing
| GETTYRomy and Jake said they wanted their parents remembered for the "incredible lives they lived and the love" they gave.
The couple were discovered on Sunday afternoon after a massage therapist arrived at the property and could not gain entry, prompting a call to Romy.
A source told the Daily Mail the pair had their throats slashed and were killed in their bed.
Nick had been residing in the guesthouse at his parents' Brentwood home.
He reportedly checked into The Pierside Santa Monica hotel around 4am on Sunday.
Staff later found the shower "full of blood" with splatters on the bed.
Sources said Reiner had grown increasingly concerned about Nick, who had a history of drug addiction and mental breakdowns.