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.

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