Officers remain in contact with the nine-month-old canine’s owners – a 32-year-old woman and her former boyfriend (36).
The Staffordshire bull terrier, known as Tinkerbell, survived the fall about 6am near the Hodges Drive exit, but its injuries proved too serious and it had to be euthanised.
RSPCA spokeswoman Anne Gabriel said inspectors were still gathering evidence.
They had viewed CCTV footage from a nearby retailer as part of the investigation.
She could not disclose the owners’ reaction to Tinkerbell’s death or if they were considered suspects.
“Because they are assisting in inquiries, the inspectors are quite sensitive about the fact that anything they share may become a barrier to the process.”
Joondalup Senior Sergeant Steve Dawson, whose officers handled the initial investigation, said police had handed responsibility of the case to the RSPCA.
The RSPCA urged anyone who may have seen or heard anything suspicious near the Eddystone Avenue overpass or Grey Road in Padbury in the early hours of Friday morning to call 1300 278 3589.