Two years after the terror attack forced Wall Street firms to disperse thousands of employees to suburbs, most of the firms have come back, either to their old homes or to new ones. But many of their workers remain in offices from Connecticut to central New Jersey.

Having been driven from their headquarters for weeks while they scrambled to restore trading ability, some of the biggest financial companies decided to spread out their operations to reduce their reliance on Manhattan's infrastructure. In several cases, the move to strengthen backup sites has cost New York significant numbers of jobs.

In White Plains, Morgan Stanley is renovating an office complex for the relocation of 1,400 employees, the largest shift of jobs out of New York City by any financial company since Sept. 11, 2001. The firm, which is based near Times Square but was the biggest tenant of the World Trade Center, is refitting the complex, built in the 1970's, as a secondary campus that could be a backup trading site in a disaster.

New York City officials hope that Morgan Stanley's move will not set off a new exodus, but others say that the scattering of employees is likely to continue.

"There is a greater consensus that it is no longer ideal to have one campus, but is preferable to have a decentralized operation with two or three central nodes," said M. Myers Mermel, chief executive of TenantWise, a real estate brokerage company in Manhattan. "That decentralization of operations is a huge seminal event in the financial services industry that is going to have repercussions for decades."

The main trade organization for Wall Street, the Securities Industry Association, says that its members moved 20,000 of their 190,000 New York City jobs out of the city in response to the attack. Only half of those jobs have returned, the association estimates, though it says it is hard to distinguish between the impact of the attack and the effects of the long stock market slump and the weakness of the economy.

"New York did lose employment as a result of 9/11, but after the first year, we started to see it stabilize," said Frank Fernandez, chief economist for the association, whose offices remain a few blocks from ground zero. "The dispersion since then has had more to do with cost savings."

New York is still recognized as the financial capital of the world, but the firms that conduct the business of buying and selling stocks, bonds and commodities have been slowly reducing their presence in the financial district in Lower Manhattan for decades, spreading their employees around the city, the region and beyond.

Before Sept. 11, 2001, a quarter of the industry's domestic jobs were in New York, down from 37 percent two decades earlier. With the scattering that followed the attack, the city's share has fallen to 20 percent.

Mr. Mermel estimated that in the last two years Lower Manhattan had lost 64,000 jobs — in finance and other areas — and that more than a third of them had wound up outside the city. That dispersal, he said, increases the likelihood that when companies resume hiring, they will place new employees elsewhere.

Morgan Stanley will have the room to do that in White Plains. Early next year, the firm plans to move 700 executives there from its brokerage operation, which was housed in the trade center and, after the attack, in Midtown offices. They will be joined later next year by another 700 employees from the company's trading operations in Midtown. Those moves will leave the firm with about 9,000 employees in New York City.

Morgan Stanley executives decided quickly after the attack, which killed 13 of the firm's 3,700 workers in the trade center, that it would be a mistake to keep operations and employees so concentrated.

They sold a nearly finished office tower in Midtown and bought the former headquarters of Texaco in White Plains. To outfit the site, the firm has hauled out acres of 1970's green carpeting and piles of shaggy rugs and started installing a trading floor with hundreds of computers and thousands of phone lines.

"Our facilities team put the Midtown campus together, so we were very saddened to have to sell that building," said George Drake, managing director for global corporate services at Morgan Stanley. "But this is the right thing to do. We were the first to come up with the concept of developing a campus within the central business district and the first to rethink it."

So far, among the other big financial firms, only Bank of New York is taking such a drastic and costly step as a direct response to the attack. The bank, which has more than 9,000 employees at its headquarters on Wall Street and two other buildings nearby, plans to move 1,500 of them to Brooklyn in the spring.

That site will also be a backup center that could house 2,000 workers in an emergency, said Don Monks, the bank's senior executive vice president for technology and operations.

In response to the terror attack, most other major Wall Street firms have established second or third sites that can be backup centers.

Lehman Brothers, whose headquarters in the World Financial Center were damaged in the attack, moved most of its staff into the Midtown building that Morgan Stanley abandoned. But the firm, which had 1,150 employees in Jersey City before the attack, has added 400 workers there while keeping its head count in New York steady at 6,000.

Other firms have merely beefed up backup centers they had before the disaster so that they can switch their trading there in an emergency.

"Building a trading floor like Morgan Stanley is doing, with false floors and state-of-the-art wiring, is not something one does lightly," said Mr. Fernandez, the economist with the Securities Industry Association. "Some firms just felt that they couldn't afford it."

Though the shutdown of the stock and bond markets on Sept. 11 was unusual, the effects of the disruption were, for the most part, temporary, Mr. Fernandez said. The main concern now is not to avert another disruption at any cost, he said, but to recover from it more quickly if another occurs.

"How fast is fast enough?" Mr. Fernandez asked. "There were people who said after 9/11 that it was probably good that the markets were closed for a period of time. The stock market only fell 7 percent when the market reopened. Had we rushed it, had we come up quicker, the sell-off may have been much worse."