Police want to hear from anyone who saw the crash, or any of the three vehicles before the crash, about 4.40am on Saturday, March 28.
The freeway was closed from Anketell Road to Thomas Road in both directions for most of that day as a result of the crash.
A white Toyota Hiace van travelling in the south-bound lanes on the freeway was struck by a Nissan semi-trailer which, in turn, hit a Holden Commodore ute.
A 62-year-old Bentley man, the passenger in the Toyota, died at the scene.
A 57-year-old Leda man, Ronald Maxwell Carter, who was driving the Nissan, later died at Fiona Stanley Hospital.
The driver of the Toyota, a 38-year-old man, was taken to the same hospital for treatment.
Major Crash investigations are expected to take some time, and will consider the road surface, visibility, weather, speed, alcohol and any other possible factors which may have contributed to the accidents.
The report will then go to the Coroner.
Opposition leader Mark McGowan last week called for action to ensure this type of accident never happened again.
�Expert internal investigations need to be completed as quickly as possible,� he said.
A Department of Fire and Emergency Services (DFES) spokeswoman said DFES was contacted, at 4.13am on Saturday, March 28, through the 000 line regarding low visibility in the area. It dispatched the Mandogalup Bush Fire Brigade to investigate.
A precautionary public information alert was issued, based on advice from the DFES Communications Centre at 4.54am, which said �there is no threat to lives or homes but there is a lot of smoke in the area�.
At 5.01am firefighters reported there were no signs of fire or smoke.
Fire and Rescue Service crews attended the crash scene, helping to pull those involved in the crash from their vehicles.
A fire believed to have been deliberately lit began at The Spectacles on Department of Parks and Wildlife land on February 5.
Fire fighters have attended flare-ups in the area, including one that prompted the partial closure of the freeway between Rowley and Mortimer roads on February 24.
Further investigations into the incident are a matter for WA Police, the DFES spokeswoman said.
Anyone with information should call Crime Stoppers on 1800 333 000.