Heat radiated off the white concrete steps and picnic tables out front of the Alameda County Courthouse where a band of would-be home-buyers gathered. An investigation by the UC Berkeley Investigative Reporting Program, relying on thousands of property records, court documents and dozens of interviews with bidders, provides a behind-the-scenes look at the murky world of foreclosure auctions, where intimidation was commonplace and millions in cashier's checks were exchanged daily. A windfall amid recessionFor some local real estate investors, the collapse of the housing market wasn't a tragedy; it was one of the greatest windfalls in real estate history. Between 2007 and 2012, banks foreclosed on nearly 150,000 homes in the Bay Area, according to Property Radar, a real estate tracking company. At least 25,000 of those houses were purchased at auctions across the region's nine counties, according to an analysis of property records, attracting bidders with plenty of cash to scoop up deals by the score. To date, 46 Bay Area real estate investors have pleaded guilty to rigging foreclosure auctions in Alameda, Contra Costa, San Francisco and San Mateo counties. Greg Casorso, a frequent bidder at the courthouse steps in Alameda County, described investors who perfected a system of buying houses at public auction. Casorso also acknowledged buying properties at a minimum asking price and then holding private auctions called rounds, where bidders flipped the properties and kept the profits from the banks - but he said he believed he was not doing anything illegal by doing so. Called collusionAccording to property records provided by CoreLogic, the average price of homes purchased by the 46 investors who have pleaded guilty was roughly $225,000, compared with an average of more than $275,000 for all houses purchased at auction across the Bay Area between 2007 and 2011. David J. Johnson, special agent in charge of the FBI San Francisco field office, said any agreement made by individuals to limit competition at public auctions is a form of collusion, a violation of federal antitrust laws. Among them is Kramer, who is able to continue to buy homes at foreclosure auctions through his Oakland real estate investment company, Robert Kramer & Associates, in the meantime. Casorso was a bidder at foreclosure auctions for Community Fund LLC in San Leandro, the largest purchaser of homes at foreclosure auctions in the Bay Area from 2007 through 2012, according to property records. Casorso said he started buying homes in 2009, when dozens of newcomers showed up on the courthouse steps in what he described as "a shark feeding frenzy." Casorso spoke of investors physically closing off the circle to potential buyers and bidding up other investors before suddenly pulling out, a practice he called "pump and dump." The investigationOn the morning of Jan. 11, 2011, Casorso said, he was returning home from a golf driving range when two FBI agents approached him on the sidewalk outside his former home in Oakland. Casorso said the agents told him they knew all about illegal activity at the courthouse steps, said it was the "biggest open secret there is," and that it was in his best interest to talk.