The son of beloved Hollywood director Rob Reiner was lying in wait before killing the filmmaker and his wife, a new indictment alleges.

Nick Reiner, 32, who is accused of fatally stabbing his parents in their Los Angeles home last December, pleaded not guilty on Wednesday to two counts of murder with a special circumstance after the new grand-jury indictment was unsealed.

The indictment, which allows prosecutors to skip California’s preliminary hearing process, did not specify details on Reiner’s alleged actions the night his parents died.

“This was a profound betrayal by someone who was loved and trusted by the very people he is accused of killing,” prosecutors said.

The grand jury indictment, returned on 20 July, amplifies the charge by adding a special circumstance allegation that the Reiner allegedly committed the murders by means of lying in wait.

The allegation means that Reiner is now eligible for the death penalty – which has been temporarily halted in the state – but prosecutors have not said if they will pursue it.

“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,” Los Angeles County District Attorney Nathan Hochman said.

Reiner also faces a special allegation that he personally used a knife in the killings, another aggravating factor because it is a dangerous and deadly weapon under California law.

He is scheduled to return to court on 15 September for a hearing.

Reiner pleaded not guilty in February to initial charges brought in the case.

Rob Reiner, 78, known for the films This is Spinal Tap, Misery and A Few Good Men, and his wife, photographer Michele Singer Reiner, 70, were found dead on 14 December in their home in the affluent neighbourhood of Brentwood in Los Angeles. Their bodies were reportedly discovered by their daughter.

The couple died from “multiple sharp force injuries”, the LA County medical examiner said at the time.

Their son, Nick, was arrested in Los Angeles the same day.

Jake Reiner, the couple’s eldest son, in April shared how his world “collapsed” after his parents died and his own brother was charged with their murders.

He wrote on Substack that “nothing compares to losing both of them at the same time and, on top of that, having your brother be at the centre of it”.