Investors who lost billions on boatloads of faulty mortgage securities have had a hard time holding Wall Street accountable for selling the things in the first place. … Because many of the worst lenders are now out of business, investors in search of recoveries have turned to the banks that packaged the loans into securities. But successfully arguing that Wall Street aided lenders in a fraud is tough under federal securities laws. This is largely a result of Supreme Court decisions barring investors from bringing federal securities fraud cases that accuse underwriters and other third parties as enablers.