His tactics have helped the Brooklyn-based investor acquire two Manhattan office towers this year for almost US$2.4 billion and become the biggest buyer of New York real estate this year.

"Werner is a wizard," said Douglas Harmon, a broker at Eastdil Secured who has worked with Werner on transactions including his US$1.5 billion deal to buy 5 Times Square in Midtown. "He plies his trade with a maniacal relentlessness that is as efficient as it is effective. He's a real quick study, and he's made billions of dollars for his investors."

The purchase by a Werner-led group of the Times Square tower, the headquarters for accounting firm Ernst & Young, was the biggest of a single building in New York since Google paid US$1.8 billion for 111 Eighth Avenue at the end of 2010, according to Real Capital Analytics. In August, a Werner partnership agreed to buy the Socony-Mobil Building across from Grand Central Terminal for US$900 million.

The two transactions make Werner the biggest buyer of all types of real estate in New York this year, according to Real Capital. SL Green Realty Corp, Manhattan's largest office landlord, is No7, while Blackstone Group, the biggest private equity real estate investor, ranks 10th, according to Real Capital data showing only completed purchases.

Ivanhoe Cambridge would eclipse Werner as the top buyer if the unit of pension fund Caisse de Depot et Placement du Quebec completes its US$2.25 billion deal for 1095 Avenue of the Americas with a partner by December 31. The purchase would bring Ivanhoe's acquisitions to US$2.4 billion, compared with US$2.37 billion for Werner.

He would still outrank China's Anbang Insurance Group with its pending US$1.95 billion purchase of the Waldorf Astoria hotel.

Werner, who lives in Brooklyn, is one of a small group of Orthodox Jews from New York's outer boroughs who trade in high-profile real estate while avoiding the spotlight. One of his strategies was to pull together groups of investors for his acquisitions, said Kenneth Patton, dean emeritus of New York University's Schack Institute of Real Estate. "He works like a broker but transacts like a principal," he said.

Patton was a broker trying to sell a Brooklyn office building that was in foreclosure when he met Werner in the early 1990s.

"I tried to slow him down, as a seller, and dampen his eagerness to buy," he said. "I was wrong. I was in the habit of doing what most people do, over-thinking the transaction. But David was just ready to go."

Werner bought the building, then resold it a few months later for US$1 million more than he paid, Patton said.

Werner is a religious man who almost never gives interviews. His determination to keep a low profile was derived from the Jewish notion of , literally "evil eye" in Yiddish, said two sources close to him.

"It means if you flaunt your success to everybody, maybe you don't deserve that kind of success," said Abraham Weisel, an adjunct professor of finance and business management at Brooklyn College.

Werner, who grew up in Manhattan's Washington Heights section and is the son of Holocaust survivors, settled in Brokklyn after he got married in his early 20s. He runs his business under the name David Werner Real Estate from a 5,000 sqft office on Third Avenue in Midtown.

He does not own his properties outright. Instead, he is a syndicator, a practice pioneered in the 1950s by New York real estate mogul Harry Helmsley. Werner looks for good investments and puts them under contract using cash he has collected from members of his circle, a group that started with Brooklyn business associates and has grown into a global network.

He takes a small equity stake in his acquisitions, usually about 10 per cent, and often seeks out other investors to take even that off his hands, the sources close to him say.

Jeffrey Citrin, the founder of investment firm Square Mile Capital Management, first ran into Werner in 1995, when Citrin was a bondholder on a group of Manhattan office properties that included 2 Broadway. The post-war building was in poor shape, with more than a million sqft of vacancies, Citrin said. Werner wanted to buy it.

Even as bondholders doubted that he could come up with the money, Werner completed the deal by bringing in another buyer, then collected a fee, Citrin said.

His method, which he still employs today, was "to be focused and industrious in ferreting out opportunities, much more often than not off-market, signing contracts secured by non-refundable deposits and invariably closing" using other investors' capital, Citrin said.

Sometimes Werner kept a minority stake, other times he recouped his deposit, plus a profit, he added.

Office rents that are among the highest in the US are supporting building prices in midtown Manhattan. If tenant demand in the market slackened, Werner's purchases of properties such as 5 Times Square and the Socony-Mobil Building might prove risky, said Barbara Denham, an economist.

"Most of the buildings he has bought are fully occupied," said Denham, a former chief economist at brokerage Eastern Consolidated who now has an independent practice. "The market is stable, job growth should stay strong through 2015 and rents should increase. That said, with Wall Street not adding jobs in significant numbers, there will likely not be a big jump in rents."

Werner got his start as an accountant and moved into real estate after one of his clients saw in his mathematic skills someone who would be adept at property investments. After a number of small transactions, the liquidation of the estate of Sylvan Lawrence, a New York investor with holdings of more than US$1 billion, gave him his first big break in the mid-1980s.

After agreeing with the estate to buy a low-rise building on Church Street in Tribeca, Werner discovered information that warranted a price reduction, which one of his partners urged him to seek. He refused, saying his word was his bond, the sources close to him said.

Lawrence's heirs remembered that, and years later offered Werner an early crack at a portfolio of four buildings, one of which was 111 Eighth Avenue, a former freight terminal that once was the headquarters of the Port Authority of New York and New Jersey.

About two years later, Werner cashed in his 10 per cent interest in all four properties, receiving about US$7.5 million. In 2010, Google paid US$1.8 billion for 111 Eighth Avenue alone, now home to more than 4,000 of the technology company's employees.

Werner's US$10.6 billion of acquisitions since 2000 stretched across the country, with more than half in Manhattan, Real Capital said. Among buildings that have passed through his hands was Credit Suisse Group's US headquarters at 11 Madison Avenue, which he bought with partners in 2003 for US$675 million.