Attorney-General Mohamed Apandi Ali in a press conference said that investigations by the country's anti-corruption agency found no evidence to show that the USD 681 million donation was "an inducement or reward for doing or forbearing to do anything in relation to his capacity as Prime Minister".
He said that no offence was committed as the money transferred into 62-year-old Najib's accounts between March and April 2013 was "given without any consideration" by the Saudi royal family as a personal donation.
He said there were no evidences to prove that Najib had any knowledge nor had he given any approval for the transfer of the monies from the account of government-owned SRC International into his personal accounts.
"Based on the facts and evidences as a whole, I as the Public Prosecutor am satisfied that no criminal offence has been committed by the Prime Minister in relation to the three investigation papers. I will return the relevant investigation papers to MACC (Malaysian Anti-Corruption Commission) today with instructions to close the three papers," he said.
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He said Najib returned USD 620 million to the Saudi royal family in August 2013 as the money was not utilised. He did not elaborate and did not say what happened to the remaining USD 61 million.
The anti-corruption agency recorded statements from witnesses, including the donor, he said.
Opposition lawmaker Tony Pua slammed Apandi's decision, saying the fact it was a personal donation does not rule out corrupt motives or transaction.
Pua said Apandi provided no new or convincing information to support his decision.
1MDB was set up in 2009 by Najib to develop new industries.
Najib had denied any wrongdoing and said the money was a donation from the Middle East. He has said that the money was "political funding".