Billionaires, tech CEOs and top members of the Republican establishment flew to a private island resort off the coast of Georgia this weekend for the American Enterprise Institute’s annual World Forum, according to sources familiar with the secretive gathering.

The main topic at the closed-to-the-press confab? How to stop Republican front-runner Donald Trump.

Apple CEO Tim Cook, Google co-founder Larry Page, Napster creator and Facebook investor Sean Parker, and Tesla Motors and SpaceX honcho Elon Musk all attended. So did Senate Majority Leader Mitch McConnell (R-Ky.), political guru Karl Rove, House Speaker Paul Ryan, GOP Sens. Tom Cotton (Ark.), Cory Gardner (Colo.), Tim Scott (S.C.), Rob Portman (Ohio) and Ben Sasse (Neb.), who recently made news by saying he “cannot support Donald Trump.” 

Along with Ryan, the House was represented by Energy and Commerce Committee Chair Fred Upton (Mich.), Rep. Kevin Brady (Texas) and almost-Speaker Kevin McCarthy (Calif.), sources said, along with leadership figure Cathy McMorris Rodgers (Wash.), Budget Committee Chairman Tom Price (R-Ga.), Financial Services Committee Chairman Jeb Hensarling (Texas) and Diane Black (Tenn.).

Philip Anschutz, the billionaire GOP donor whose company owns a stake in Sea Island, was also there, along with Democratic Rep. John Delaney, who represents Maryland. Arthur Sulzberger, the publisher of The New York Times, was there, too, a Times spokeswoman confirmed. 

“A specter was haunting the World Forum–the specter of Donald Trump,” Kristol wrote in an emailed report from the conference, borrowing the opening lines of the Communist Manifesto. “There was much unhappiness about his emergence, a good deal of talk, some of it insightful and thoughtful, about why he’s done so well, and many expressions of hope that he would be defeated.”

“The key task now, to once again paraphrase Karl Marx, is less to understand Trump than to stop him,” Kristol wrote. “In general, there’s a little too much hand-wringing, brow-furrowing, and fatalism out there and not quite enough resolving to save the party from nominating or the country electing someone who simply shouldn’t be president.”

A highlight of the gathering was a presentation by Rove about focus group findings on Trump. The business mogul’s greatest weakness, according to Rove, was that voters have a very hard time envisioning him as “presidential” and as somebody their children should look up to. They also see him as somebody who can be erratic and shouldn’t have his (small) fingers anywhere near a nuclear trigger.

Sources familiar with the meeting — who requested anonymity because the forum is off the record — said that much of the conversation around Trump centered on “how this happened, rather than how are we going to stop him,” as one person put it.

Trump, who already has nearly one-third of the delegates he needs to secure the GOP nomination, faces major tests in the Florida and Ohio primaries next week. If he wins both those states, he will need to win just half of the remaining delegates to secure the nomination.

He wasn’t the only topic of the wide-ranging conference, however. At one point, Cotton and Apple’s Cook fiercely debated cell phone encryption, a source familiar with the exchange told HuffPost. “Cotton was pretty harsh on Cook,” the source said, and “everyone was a little uncomfortable about how hostile Cotton was.” (Apple is in the midst of a battle with the Justice Department and the FBI over an encrypted iPhone that belonged to one of the San Bernardino shooters.) Cook did not attend the Rove session, or otherwise take part in any political organizing, emphasized a source close to Cook.

AEI has held the annual forum on Sea Island for years. It’s so secret that in 2015, Bloomberg News complained that no one would even say whether it had snowed. 

Federal Aviation Administration records available on FlightAware.com show that a fleet of private jets flew into and out of two small airports near Sea Island this weekend. Fifty-four planes flew out of the airport on St. Simons Island, Georgia, on Sunday — nearly four times as many as departed from the airport the previous Sunday. 

Many of the planes are registered to jet-sharing companies such as NetJets and Flexjet or private jet services companies such as Jetsetter. At least two of them flew directly to San Jose, California, home of many tech giants, on Sunday.

Another plane, which arrived from Eaton, Colorado, on Wednesday and flew back there on Sunday, is registered to Monfort Aviation, LLC, a private, tax-exempt trust. FAA records don’t indicate who controls Monfort Aviation, but it shares a name with Dick and Charlie Monfort, the Colorado-based heirs to a meatpacking fortune who now own the Colorado Rockies baseball team. The plane, a Raytheon Hawker 800XP, seats 15 people. Anschutz, the billionaire whose company part owns Sea Island, is also from Colorado. 

Another private plane, a Canadair Challenger, flew cross-country from St. Simons to Van Nuys Airport in Southern California on Friday. Van Nuys Airport is so associated with millionaires and billionaires that their disputes over space at the field occasionally spill into the news media.

Another plane, a tri-jet Dassault Falcon 900, flew into St. Simons on Thursday from Westchester County, New York, and returned on Sunday. It’s registered to Northwood Investors LLC, which is run by John Kukral, whose official bio notes he’s been involved in real estate deals worth over $40 billion.

“The event is private and off-the record, therefore we do not comment further on the content or attendees,” said Judy Stecker, a spokeswoman for AEI. She described the forum as “an informal gathering of leading thinkers from all ideological backgrounds to discuss challenges that the United States and the free world face in economics, security and social welfare.”

Sea Island Resort — which boasts three golf courses and a spa and fitness center that, at 65,000 square feet, would fill nearly two-thirds of a Home Depot — is famous for its isolation. It’s surrounded by marshes and some distance from the nearest large commercial airports. In 2004, when President George W. Bush hosted the annual G-8 summit on the island, the press center for the event was located 80 miles away in Savannah, Georgia.

The Anschutz Corp., Starwood Capital Group Global, Avenue Capital Group, and Oaktree Capital Management bought the then-bankrupt resort — which covers the entire island — in 2010 for $212.4 million.

“It is not much of a place to experience average America,” The New York Times wrote of Sea Island in 2004. “But it is a fine locale to shut out the rest of the world, view conspicuous architectural consumption and walk beaches that have little or no public access.”

In 2015, AEI’s Sea Island gala drew most of the men who would become the Republican party’s presidential candidates, according to an agenda Bloomberg obtained at the time. Scheduled speakers included former Florida Gov. Jeb Bush, New Jersey Gov. Chris Christie, Texas Sen. Ted Cruz, South Carolina Sen. Lindsey Graham, Louisiana Gov. Bobby Jindal, and former Pennsylvania Sen. Rick Santorum. (Some scheduled speakers may not have attended; a snowstorm snarled transportation up and down the East Coast that weekend.)

AEI paid $32,490.97 for 11 members of Congress to attend the conference in 2015 alone, according to disclosure records available on Legistorm.com.

Democratic officials, including Sen. Cory Booker (D-N.J.), Jason Furman, the chair of Barack Obama’s Council of Economic Advisers, Gene Sperling, another top Obama economic adviser, and former Treasury Secretary Larry Summers, were listed as 2015 attendees, Bloomberg reported at the time.

Christie was scheduled to deliver the opening remarks at the conference that year.

A few weeks ago, he endorsed Trump.