In 2017, Michael Moritz, the billionaire venture capitalist, sent a note to a potential investor about what he described as an unusual opportunity: a chance to invest in the creation of a new California city.

The site was in a corner of the San Francisco Bay Area where land was cheap. Mr. Moritz and others had dreams of transforming tens of thousands of acres into a bustling metropolis that, according to the pitch, could generate thousands of jobs and be as walkable as Paris or the West Village in New York.

He painted a kind of urban blank slate where everything from design to construction methods and new forms of governance could be rethought. And it would all be a short distance from San Francisco and Silicon Valley. “Let me know if this tickles your fancy,” he said in the note, a copy of which was reviewed by The New York Times.

Since then, a company called Flannery Associates has been buying large plots of land in a largely agricultural region 60 miles northeast of San Francisco. The company, which has little information public about its operations, has committed more than $800 million to secure thousands of acres of farmland, court documents show. One parcel after another, Flannery made offers to every landowner for miles, paying several times the market rate, whether the land had been listed for sale or not.

The purchases by a company that no one in the area had heard of and whose business was a mystery have become the subject of heavy speculation and developing news stories, rattling landowners, local supervisors, the nearby Air Force base and members of Congress. Was Disney buying it for a new theme park? Could the purchases be linked to China? A deepwater port?

Flannery is the brainchild of Jan Sramek, 36, a former Goldman Sachs trader who has quietly courted some of the tech industry’s biggest names as investors, according to the pitch and people familiar with the matter. The company’s ambitions expand on the 2017 pitch: Take an arid patch of brown hills cut by a two-lane highway between suburbs and rural land, and convert into it into a community with tens of thousands of residents, clean energy, public transportation and dense urban life.

The company’s investors, whose identities have not been previously reported, are a who’s who of Silicon Valley, according to three people who were not authorized to speak publicly about the plans.

They include Mr. Moritz; Reid Hoffman, the LinkedIn co-founder, venture capitalist and Democratic donor; Marc Andreessen and Chris Dixon, investors at the Andreessen Horowitz venture capital firm; Patrick and John Collison, the sibling co-founders of the payments company Stripe; Laurene Powell Jobs, founder of the Emerson Collective; and Nat Friedman and Daniel Gross, entrepreneurs turned investors. Andreessen Horowitz is also a backer. It was unclear how much each had invested.

Brian Brokaw, a representative for the investor group, said in a statement that the group was made up of “Californians who believe that Solano County’s and California’s best days are ahead.” He said the group planned to start working with Solano County residents and elected officials, as well as with Travis Air Force Base, next week.

In California, housing has long been an intractable problem, and Silicon Valley’s moguls have long been frustrated with the Bay Area’s real estate shortage, and the difficulty of building in California generally, as their work forces have exploded. Companies like Google have clashed with cities like Palo Alto and Mountain View over expanding their headquarters, while their executives have funded pro-development politicians and the “Yes in my backyard” activists who have pushed for looser development and zoning laws in hopes of making it easier to build faster and taller.

The practical need for more space has at times morphed into lofty visions of building entire cities from scratch. Several years ago, Y Combinator, the start-up incubator, announced an initiative with dreams of turning empty land into a new economy and society. Years before that, Peter Thiel, the PayPal co-founder and billionaire Facebook investor, invested in the Seasteading Institute, an attempt to build a new society on lily pad-like structures in the law-and-tax-free open ocean.

But while these ideas have garnered lots of attention and curiosity — lauded in some corners for vision and derided in others for hubris — they have been little more than talk. As Flannery began seeking property, it bought so much land, so fast, that it spooked locals who had no idea who the buyer was or the plans it had in mind. Catherine Moy, the mayor of Fairfield, Calif., started posting about the project on Facebook several years ago after she got a call from a farmer about some mystery buyer making offers throughout the county. In an interview, Ms. Moy said she had gone to the county assessor’s office and found that Flannery had purchased tens of thousands of acres.

John Garamendi, a Democrat who along with Mike Thompson, another Democrat, represents the surrounding region in Congress, said he had been trying to figure out the company’s identity for four years.

“I couldn’t find out anything,” he said.

On Friday, he said that had suddenly changed. This week representatives for Flannery reached out to him and other elected officials requesting meetings about their plans. That meeting is now being scheduled, he said.

“This is their first effort, ever, to talk to any of the local representatives, myself included,” he said. The land that Flannery has been purchasing is not zoned for residential use, and even in his 2017 pitch, Mr. Moritz acknowledged that rezoning could “clearly be challenging” — a nod to California’s notoriously difficult and litigious development process.

To pull off the project, the company will almost certainly have to use the state’s initiative system to get Solano County residents to vote on it. The hope is that voters will be enticed by promises of thousands of local jobs, increased tax revenue and investments in infrastructure like parks, a performing arts center, shopping, dining and a trade school.

The financial gains could be huge, Mr. Moritz said in the 2017 pitch. He estimated the return could be many times the initial investment just from the rezoning, and far more if and when they started building.

“If the plans materialize anywhere close to what is being contemplated, this should be a spectacular investment,” Mr. Moritz wrote.

The Bay Area is among the country’s most expensive regions, even after rent and home prices fell after the pandemic. Economists and housing experts have for decades blamed a longstanding housing shortage and California’s inability to build enough to meet demand.

Mr. Moritz nodded to this in the email to the investor, arguing that “this effort should relieve some of the Silicon Valley pressures we all feel — rising home prices, homelessness, congestion etc.” He added that his group had secured some 1,400 acres for less than $5,000 per acre. The price per acre has since escalated, and the company’s most recent purchases have neared $20,000 per acre, according to court documents and people familiar with the matter.

The purchases burst into public view this spring when lawyers for Flannery filed a lawsuit in U.S. District Court, accusing landowners of colluding to inflate prices.

The group focused on Jepson Prairie and Montezuma Hills, an agricultural patch of eastern Solano County between the cities of Fairfield and Rio Vista, according to the lawsuit. This area is mostly unpopulated and covered with ranches, windmills and power lines.

In November 2018, the company sent offers to “most landowners in this area,” the lawsuit said, and included incentives such as allowing sellers to retain income from wind turbines, as well as stay on the properties rent-free under long-term lease-back agreements. Over the five years, the company purchased some 140 properties from 400 owners, the lawsuit said. This month, a lawyer representing landowners jointly filed a motion to dismiss the case. In July, three landowners said they had reached a potential settlement with Flannery. Other owners could not be reached for comment this week, or had declined to do so.

As the offers continued and prices escalated, landowners in Solano County started buzzing about who was buying so much land for so much money.

“They would come with an offer of four and five times over the market at the time,” Ms. Moy said. “They were deals that they couldn’t pass up.”

Flannery’s offers were creating multimillionaires across the county, but no one seemed to know what the mysterious company intended to do with land that now amounted to a large chunk of the entire county.

That changed last week, when residents started receiving texts and emails with a poll gauging their opinions on a number of questions. One asked them to rate the favorability of several names including “Joe Biden,” “Donald Trump” and “Flannery Associates.” Another question began with a description of a possible ballot initiative for a project that “would include a new city with tens of thousands of new homes, a large solar energy farm, orchards with over a million new trees, and over 10,000 acres of new parks and open space.” Ms. Moy cited poor infrastructure, including the two-lane highway bisecting the region that she said was already clogged by super-commuters driving to the edges of the Bay Area and beyond. The area is also prone to regular droughts and is at high risk for wildfires.

“It seems very pie in the sky,” she said.