London (CNN Business) Saudi Arabia's isolation in the global business world deepened Tuesday as three of Europe's top bankers joined a growing list of executives who have pulled out of a high-profile investment conference in Riyadh next week.

A spokesperson for HSBC ( HSBC ) said its CEO, John Flint, would not attend the Future Investment Initiative, and a person familiar with the situation said Credit Suisse ( CS ) CEO Tidjane Thiam would no longer be going.

Both executives were previously listed as speakers and both banks were strategic partners for the event. Neither bank would comment on the status of the partnerships.

The exodus began last week amid mounting questions about Saudi Arabia's role in the disappearance of journalist Jamal Khashoggi at its consulate in Istanbul on October 2.

Sources told CNN on Monday that Saudi Arabia is preparing to acknowledge that Khashoggi was killed during an interrogation that went wrong . Saudi authorities have so far maintained that the Washington Post columnist left the consulate the same day he went in, but they have provided no evidence to support the claim.

HSBC CEO John Flint and Credit Suisse CEO Tidjane Thiam have joined the exodus from 'Davos in the desert.'

Corporate America has been leading the exodus from the Future Investment Initiative, also known as "Davos in the desert."

Uber CEO Dara Khosrowshahi and Viacom ( VIAB ) CEO Bob Bakish dropped out last week, when most of the event's international media partners , including CNN, also withdrew.

They were followed Monday by the top executives of JPMorgan Chase ( JPM ), Blackrock ( BLK ), Blackstone, MasterCard ( MA ) and Ford ( F ).

The conference was used a year ago by Crown Prince Mohammed bin Salman as a showcase for Vision 2030, his plan to modernize Saudi Arabia's economy and end its dependence on oil.

But the unexplained disappearance of Khashoggi, a former Saudi government adviser turned critic, has already prompted other business partners to cut ties with projects that are central to bin Salman's dreams.

"What has reportedly happened in Turkey ... if proved true, would clearly change the ability of any of us in the West to do business with the Saudi government," Branson said in a statement last week.

A second flagship project — a futuristic zero-emissions mega city known as NEOM — is also being shunned.

Dan Doctoroff, CEO of Alphabet ( GOOGL ) subsidiary Sidewalk Labs; Silicon Valley entrepreneur Sam Altman; Tim Brown, CEO of design company IDEO; former US Energy Secretary Ernest Moniz; and former European Commissioner Neelie Kroes have all said they won't work with NEOM despite being named as senior advisers.

The organizers of the Riyadh conference said Monday that they were disappointed about the cancellations but were still expecting thousands of speakers, moderators and guests to attend.

The chief executives of several prominent Asian and European companies — some of whom have benefited from Saudi investments — are still planning to attend, or are refusing to talk about their plans.

Most important among them is Masayoshi Son, founder and CEO of SoftBank ( SFTBF ). The Japanese company has become one of the world's most powerful tech investors, thanks to Saudi money.

SoftBank has also committed to help the Saudi crown prince in his efforts to overhaul the economy. It's involved in NEOM and a separate Saudi project to build the world's largest solar farm.

"This is [a] fantastic opportunity," Son said last year, speaking about the mega-city project as he sat next to bin Salman. "Because of the leadership, because of the location, because of the people in the kingdom, we can make it happen."

Shares in SoftBank slumped 7% on Monday as investors worried that the close relationship could hurt the company. The stock recovered some of those losses on Tuesday.