Attorney-general Mohamed Apandi Ali's decision that there had been no wrongdoing in the RM2.6 billion donation scandal cannot be accepted unless the money trail is shown, Dr Mahathir Mohamad says.

"The mere letter of admission by the generous donor is not enough. Neither can the decision of the attorney-general that there has been no wrongdoing be accepted without all the records aforementioned," the former prime minister wrote on his blog today.

In January 2016, Apandi had declared that Prime Minister Najib Abdul Razak had done no wrong in the donation scandal, involving a US$681 million transfer into his personal bank accounts.

The attorney-general had also said that based on documents submitted by the Malaysian Anti-Corruption Commission (MACC), the sum was a donation from a member of the Saudi royal family.

Apandi went on to say that Najib had returned US$620 million to the donor. 

However, this claim has been disputed by Najib's critics, who cite the US Department of Justice's ( DOJ) civil forfeiture suits involving 1MDB. 

The DOJ suits claim that the US$681 million that went to Najib, dubbed as Malaysian Official 1 (MO1) in their documents, was misappropriated from 1MDB.

Najib's identity as MO1 was confirmed by Minister in the Prime Minister's Department Abdul Rahman Dahlan. 

The DOJ further claims that the US$620 million allegedly returned had gone to Tanore Finance Corporation's bank account in Singapore. 

Singapore courts have established that the Tanore account was controlled by Penang-born tycoon Jho Low.

"Clearly an investigator can follow the records of the movements of the US$681 million, including the return of US$620 million to the generous Saudi prince. 

"Banking secrecy should not be in the way of an investigation by government authorities investigating criminal acts," Mahathir said. 

Najib has denied misappropriating 1MDB funds, saying such allegations were made by those wishing to topple him.