Interviewed by Reuters the same day, Mahathir said authorities were looking to bring cases ranging from embezzlement, bribery with government money and abuse of power against his former protege.
Mahathir told Reuters investigators had found Najib's signature on cheques and documents. Reacting to Najib's protestations of innocence, Mahathir told a newspaper that Najib had lost credibility.
“Who wants to believe him that he didn’t know, when he signed Every bit of money that goes in and out of the first borrowing of 42 billion ringgit ($10.5 billion), all his signature," Mahathir said in an interview with the Malay Mail.
The amount refers to the total debt accumulated by 1MDB, a fund founded by Najib in 2009.
“If he doesn’t know, it must be that he doesn’t understand what a signature means... Najib always assume that people are stupid."
Najib, in some of his most extensive comments yet on the 1MDB scandal, had told Reuters that he did not know if hundreds of millions of dollars that moved through his personal account was from 1MDB, and if money from the fund was eventually laundered to acquire assets globally, including yachts, paintings, gems and prime real estate.
Lim Kit Siang, a senior leader in Mahathir's coalition, said there was no way those running 1MDB could have kept the alleged embezzlement of billions of dollars in public funds a secret from Najib, especially after the U.S. Department of Justice filed civil suits that detailed how and where some of the money moved.
"All Malaysians should study and dissect the most astonishing interview given by Najib on the 1MDB scandal with Reuters," Lim said.
"I do not believe that Najib is so stupid, incompetent or ignorant, which only means he was lying through his teeth in his interview with Reuters," he added.
Najib also said the nearly 300 boxes of designer handbags seized by police during searches were mainly gifts given to his wife and daughter and had nothing to do with 1MDB.
Najib's denials sparked outrage among Malaysians who roasted him on social media.
"Mr Ex Pm you know nothing except for lavish lifestyle...," read a tweet posted under the handle @mase99rati.
Transactions involving 1MDB are being investigated in half a dozen countries, including the United States, where it has become the biggest case pursued by the Department of Justice under its anti-kleptocracy programme.
The DoJ has alleged in lawsuits that more than $4.5 billion from 1MDB was laundered through a complex web of transactions and shell companies, of which $681 million ended in Najib's bank account. Najib has said the money in his account were donations from Saudi Arabia, most of which he returned.
According to the DoJ, assets purchased using 1MDB money include a Picasso painting, luxury real estate in South California and New York, shares in a Hollywood production company and a $265 million yacht, and more than $200 million worth of jewellery - including a 22-carat pink diamond pendant and necklace. ($1 = 4.0140 ringgit)
(Reporting by Joseph Sipalan; Editing by Simon Cameron-Moore) ((Joseph.Sipalan@thomsonreuters.com; +60323338019; Reuters Messaging: email@example.com))