The grand jury, which had been meeting in secret, returned the indictment on July 20, and it was unsealed on Wednesday, when 32-year-old Nick Reiner entered a not guilty plea.
The director, actor and Hollywood luminary Rob Reiner and his wife, photographer and producer Michele Singer Reiner, were stabbed to death in their home in the upscale Brentwood section of Los Angeles on December 14. Nick Reiner was arrested hours later and charged two days later.
The grand jury indictment is a major step toward trial and allows prosecutors to skip California's preliminary hearing process, where evidence is publicly aired.
"This was a profound betrayal by someone who was loved and trusted by the very people he is accused of killing," Los Angeles County District Attorney Nathan Hochman said in a statement.
"We hope that by having a grand jury return an indictment in this case it will bring us one step closer to a trial and achieving justice."
The unsealed indictment reveals few new details in a case that has been shrouded in secrecy. But the lying in wait allegation along with other circumstances mean Reiner is eligible for the death penalty. Prosecutors say they haven't decided whether to seek it.
The indictment did not say how or why Reiner was lying in wait. It also includes an allegation that he used a knife in the killing. Authorities previously said the couple was stabbed. Many of the most basic questions about the killing remain unanswered publicly.
Prosecutors have said nothing about possible motives, and leaks in the case have been virtually nonexistent on both sides.
The Los Angeles County chief medical examiner told The Associated Press he would like to make their autopsy reports public, but a court order has prevented him from doing so.
The indictment comes as Reiner is seeking unpaid money from a trust his parents established for him, saying he needs it to help in his defence.
Reiner said in a court petition that the trustees who oversee the funds have denied them to him without legal justification. The money was to have gone to Nick Reiner even if his parents were alive, legal filings say. The petition says the trust has at least $US1.5 million ($A2.1 million) in assets, but trustees would not share the exact amount.
He is seeking to hire back high-profile criminal defence lawyer Alan Jackson, who initially represented him before Reiner's siblings would no longer agree to fund his defence and a public defender took over.
A hearing on the petition is set for later in August.
The LA County Public Defender's Office, which is representing Reiner, declined to comment on the indictment.
California prosecutors can charge a defendant and take the case through a preliminary hearing — a sort of trial in miniature where evidence is publicly aired and a judge decides whether it should be sent to trial — as recently happened with singer D4vd.
Or they can use a grand jury to go through the process in secret. They can also publicly file charges and later seek a secret grand jury indictment, essentially creating a new version of the case, as happened here and in the Los Angeles prosecution of Harvey Weinstein.
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