The last time the housing market was this hot in Phoenix and Las Vegas, the buyers pushing up prices were mostly small time. Nowadays, they are big time — Wall Street big.

Large investment firms have spent billions of dollars over the last year buying homes in some of the nation’s most depressed markets. The influx has been so great, and the resulting price gains so big, that ordinary buyers are feeling squeezed out. Some are already wondering if prices will slump anew if the big money stops flowing.

“The growth is being propelled by institutional money,” said Suzanne Mistretta, an analyst at Fitch Ratings. “The question is how much the change in prices really reflects market demand, rather than one-off market shifts that may not be around in a couple years.”

Wall Street played a central role in the last housing boom by supplying easy — and, in retrospect, risky — mortgage financing. Now, investment companies like the Blackstone Group have swooped in, buying thousands of houses in the same areas where the financial crisis hit hardest.

Blackstone, which helped define a period of Wall Street hyperwealth, has bought some 26,000 homes in nine states. Colony Capital, a Los Angeles-based investment firm, is spending $250 million each month and already owns 10,000 properties. With little fanfare, these and other financial companies have become significant landlords on Main Street. Most of the firms are renting out the homes, with the possibility of unloading them at a profit when prices rise far enough.

While these investors have not touched many healthy real estate markets, they are among the biggest buyers in struggling areas of the country where housing prices have been increasing the fastest. Those gains, in turn, have been at the leading edge of rising home prices nationwide.

Some see the emergence of Wall Street buyers as a market-driven answer to the nation’s housing ills. Investment companies are buying up rundown homes at a time when ordinary people can’t or won’t.

Nationwide, 68 percent of the damaged homes sold in April went to investors, and only 19 percent to first-time home buyers, according to Campbell HousingPulse. That is helping to shore up prices and create confidence in the broader markets.

“When people write the story of this housing recovery, these investors will be seen to have helped put the floor under the housing market,” said David Bragg, an analyst at Green Street Advisors. “In some of the key markets, that contributed to the recovery.”

The story, though, often looks more complicated on the ground. Joe Cusumano, a real estate agent in Riverside County, Calif., said that in recent months 90 percent of his business had been for companies like Invitation Homes, a Blackstone subsidiary. Home values in Riverside County have risen by 15 percent in the last year, according to CoreLogic.

But Mr. Cusumano said he wondered if faraway investors would properly maintain the homes they buy. He said that Invitation Homes had been willing to put money into the properties, but he was not so sure about the other players. He also worries what will happen when these investors start selling, as they inevitably will.

Behind the Rise in House Prices, Wall Street Buyers – NYTimes.com.