The Malaysian Anti-Corruption Commission (MACC) will "most likely" appeal the attorney-general's decision to close the RM2.6 billion and SRC International cases.
The commission's special operations director Bahri Mohd Zin described the cases to The Star as being "straightforward".
Yesterday, attorney-general Mohamed Apandi Ali said there were no grounds to prosecute Prime Minister Najib Abdul Razak in relation to the cases.
Among others, Apandi said the RM2.6 billion donation in 2013 came from the Saudi royal family and of this, RM2.03 billion was returned in August the same year.
Bahri, who spearheaded the SRC probe, was among the MACC directors slapped with a transfer to the Prime Minister's Department last year.
This was after he swore to find the culprits amid a crackdown on the MACC officers involved in the probe.
The transfer order was subsequently rescinded.
The crackdown was initiated after the Wall Street Journal and whistleblower website Sarawak Report exposed the transactions based on leaked documents by Malaysian investigators.
Apandi's decision not to take action against Najib also drew condemnation from various quarters, with some claiming it was for this reason he had been appointed.
The AG took over the post weeks after the donation and SRC issues erupted following his predecessor Abdul Gani Patail's sudden removal over health reasons.
However, Apandi and the government have denied any ulterior agenda, and the AG had pledged to act without fear or favour.
Najib has repeatedly denied abusing public funds for personal gain, and blamed such allegations on those plotting his downfall.