NEW YORK (NYTIMES) - It seemed a facetious question, one intended to provoke the star witness: "Do you think you are good at lying?"

But it is the crucial issue at the centre of what is likely to be the only trial on US soil in one of the largest international kleptocracy cases in history, the looting of billions of dollars from the people of Malaysia.

A former banker at Goldman Sachs, Roger Ng, is accused of taking part in a bribery and kickback scheme that enabled the fraud, which plundered more than US$4 billion (S$5.5 billion) from a Malaysian sovereign wealth fund and bought a king's ransom of jewellery, art and real estate from New York City to London to Beverly Hills, California.

Ng's former boss at Goldman, Tim Leissner, was for years one of Goldman's most powerful deal-makers in Asia.

Now, he is the government's key witness - and an admittedly prolific fabricator.

On the stand in a Brooklyn federal courtroom, he acknowledged misleading co-workers, investigators and all three of his wives.

But when Ng's lawyer prodded him with the question about whether he is a good liar, Leissner coolly replied: "I don't think so."

Leissner's 10 days of testimony - including six days of blistering cross-examination - laid out the details of a global fraud that toppled Malaysia's prime minister and forced Goldman Sachs, one of the world's most prestigious financial institutions, to go before a US judge and admit, for the first time in its 153-year-history, that it was guilty of a crime.

The question for jurors: How much of what Leissner said was true?

Leissner acknowledged to the court that he has "lied a lot," including by presenting a bogus divorce decree to his now-estranged wife, model and fashion designer Kimora Lee Simmons, so she would marry him eight years ago.

But he insists he is telling the truth about Ng, whom prosecutors say helped line the pockets of officials in Abu Dhabi, United Arab Emirates, and powerful Malaysians close to then-prime minister, Najib Razak.

The looted money from the 1Malaysia Development Berhad, or 1MDB, sovereign wealth fund financed a vast spending spree: Paintings by van Gogh and Monet, a yacht that was docked in Bali, a private jet and a clear acrylic grand piano given to a supermodel.

It financed a boutique hotel in Beverly Hills, bought a share of the EMI music publishing portfolio and helped make The Wolf of Wall Street, the Martin Scorsese movie about Wall Street corruption that netted Leonardo DiCaprio a Golden Globe.

The trial could be the scandal's final act.

Goldman Sachs has paid US$5 billion in fines and pleaded guilty on behalf of an Asian subsidiary.

Leissner pleaded guilty and is awaiting sentencing. Najib was convicted in his home country and sentenced to 12 years in jail.

The only other central character of the plot is unlikely to ever face trial: Jho Low, the brash financier accused of being the scheme's architect, is a fugitive believed to be living in China, beyond the reach of US prosecutors.

Leissner said Low was the "decision-maker" on all things involving 1MDB but that Ng was his primary contact at Goldman, which earned roughly US$600 million in fees to arrange the US$6.5 billion in bond deals for the fund.

"Roger made him one of his clients," Leissner said.

He testified that Ng had set up many of the meetings to plan the scheme, including one at Low's London apartment during which Low drew boxes on a piece of paper with the names of all the officials that would get bribes and gifts.

For helping arrange the payments, Leissner said, he raked in more than US$80 million. Prosecutors contend that Ng's share was US$35 million.

Ng's attorneys say that Leissner has exaggerated and distorted facts to please prosecutors and get a lighter sentence.

The money Ng received, they argue, was to repay a legitimate debt one of Leissner's wives owed Ng's wife.

"I am set to show this jury he is a liar," Ng's lead lawyer, Mr Marc Agnifilo, told Judge Margo Brodie, the chief judge for US District Court for the Eastern District of New York, during a break in the proceedings.

He described Leissner as "deceptive" and "cunning." "There aren't a lot of guys like him," Mr Agnifilo said.

Leissner's credibility has been an issue since the investigation began.

In a campaign to cast Leissner as a rogue employee, the bank told regulators and law enforcement officials that he was a master con man who had fooled everyone at the bank.

Ms Rebecca Roiphe, a former prosecutor and a professor at New York Law School who specialises in legal ethics, said it could be tricky to rely on such a witness, but "it isn't a fatal blow".

She said a prosecutor can argue that a witness is "a horrible person" and "a serial liar". "That can work when you have really bad people who have lied a lot," she said.

Mr Agnifilo got Leissner to elaborate on how he - not Ng - oversaw the payment of most of the bribe money.

He highlighted Leissner's close relationship with Low by getting Leissner to confirm his attendance at a star-studded 31st birthday party that Low arranged for himself in Las Vegas in 2012. (Leissner said Ng had also attended, although Ng was not on the guest list.)

Mr Agnifilo also had Leissner recount the many ways he deceived his wives, particularly Ms Simmons.

Leissner admitted that he had used an email account in the name of his second wife, Ms Judy Chan, to communicate with Ms Simmons while dating her, and that he was still married to Ms Chan when he and Ms Simmons were wed. (Leissner was also legally married to another woman when he married Chan.)

And Mr Agnifilo poked holes in Leissner's accounts of details of the 1MDB scheme, particularly the pivotal meeting at Low's apartment.

He pressed Leissner on why he told the FBI after his June 2018 arrest that a Morgan Stanley banker had also been present.

Leissner said he did not recall saying that and did not know why it would be in an FBI agent's write-up of the interview.

When shown a copy of the FBI report, Leissner responded: "I don't know who wrote this or how that report was written."

Mr Agnifilo also grilled Leissner - who was interviewed by law enforcement roughly 50 times before and after his guilty plea in August 2018 - about how he had trickled out details of the meeting.

Leissner had no explanation for why he did not mention Low's writing down the names of people on a chart until an interview with the FBI in April last year.

When forced to discuss some of his deceptions, Leissner sounded almost sheepish, including when he testified about using some of the looted money to buy a US$10 million house for one of his girlfriends so she would not go to authorities.

Leissner said he was not proud of his untruths, and had lied to the FBI after his arrest because he was scared. Finally, though, he had "come clean," he said.

Ng's trial is scheduled to go on for at least two more weeks, and it is hard to know just how Leissner's testimony will factor in the jury's deliberations. The government is calling other witnesses to testify about how both men violated Goldman rules in seeking the lucrative bond underwriting business, and to review bank records for Ng and his wife.

Even if the jury trusts Leissner's testimony, the litigation may not be over.

The trial had to be paused for several days to allow Ng's lawyers time to review tens of thousands of emails and other private documents belonging to Leissner that the prosecution did not deliver until after the trial began.

Prosecutors called the delay "inexcusable".

Mr Agnifilo has said the government's failure to produce the documents in a timely fashion impeded his ability to prepare for the trial. Legal experts have said the issue could give Ng grounds for an appeal if convicted.