Liberal Democrat leader Willie Rennie has pressed the Lord Advocate for an update on an investigation into the fatal M9 crash.
Mr Rennie has written to Frank Mulholland asking for details of the independent probe he instructed the Police Investigations and Review Commissioner (Pirc) to carry out.
Lamara Bell, 25, and John Yuill, 28, died after lying in a crashed car for three days after it was first reported to police last July.
Ms Bell was critically injured in the crash off the M9 near Stirling and died later in hospital. Her partner Mr Yuill, 28, died at the scene.
The Pirc investigation centres on why a telephone call made to Police Scotland on Sunday July 5, which reported their car was off the road, was not followed up.
It is also examining why a missing person inquiry relating to the pair was not linked with the information received in the call.
An interim report was submitted to the Lord Advocate on November 30.
In his letter to Mr Mulholland, Mr Rennie said: “I would be grateful for any indication that you can provide as to when this investigation will conclude and when you will be in a position to publish its findings.
“I would also appreciate it if you could outline the range of options that will be available to you once you have received the final report from the Pirc.
“As you will appreciate, once all the facts have been established, there are compelling reasons for information to be made available as soon as possible.
“And I hope that you agree with me about the need for as much transparency as possible.”
A Pirc spokesman said: “To determine the events leading up to the deaths of John Yuill, 28, and Lamara Bell, 25, all aspects of call handling and the missing persons investigation are being considered by Pirc investigators.
“The inquiry is ongoing and the Pirc continues to have regular contact with the families involved through our family liaison staff.
“The commissioner submitted an interim report to the Lord Advocate on November 30 and we are undertaking further investigations on his behalf as required.
“All findings from the investigation will be passed to the Lord Advocate for his consideration in due course.
“As in all Crown Office directed investigations, the Police Public Order and Criminal Justice (Scotland) Act 2006 does not authorise the commissioner to publish a report.”
A Crown Office spokeswoman said: “The Lord Advocate has received the letter from Willie Rennie and will respond in due course.”