Lamara Bell, 25, was found critically injured in a car three days after it crashed last July and died later in hospital.
Her partner John Yuill, 28, was found dead in the car.
Police had failed to follow up a call received about the crash.
The couple were eventually discovered in the vehicle, close to the motorway at Bannockburn on 8 July, 2015.
The case was subsequently referred to the Police Investigations and Review Commissioner (Pirc).
The Lord Advocate will consider the findings of the Pirc's supplementary report before deciding what action to take.
Outcomes could include initiating criminal proceedings or ordering a fatal accident inquiry to be held.
A Pirc spokesman said: "The commissioner's investigators pursued numerous lines of inquiry, including why a telephone call made to Police Scotland on 5 July, 2015, which reported a car off the road, was not followed up.
"The commissioner also examined the procedures used by Police Scotland to log the particular call made on 5 July, 2015.
"The investigation also looked at the robustness of Police Scotland's missing person enquiry and why this was not linked with the information received in the telephone call."
The spokesman said "many different facets" of the incident had been explored.
He said: "The commissioner recognises that it has been a difficult period for the families of Mr Yuill and Ms Bell whilst the investigation has been ongoing and has appreciated the opportunities to keep them informed of progress through our family liaison support."
A statement on behalf of the Bell family said: "The pain of losing Lamara in such tragic circumstances will never go away, but with the completion of this supplementary report from the Police Investigations and Review Commissioner, we are another step closer to knowing what happened to Lamara and John on that fateful day."
A statement from the Yuill family said they were "pleased" that the supplementary report had been submitted.
It added: "It will hopefully take us nearer to some insight or answers as to what happened with John and Lamara on 5th July last year.
"We would again ask for our privacy to be respected and to allow all those involved in the investigation to continue to do their jobs."
The findings of an "exhaustive investigation" into the circumstances of the M9 crash which killed two people have been submitted to the Lord Advocate.