The cause of last week's stock market plunge is still a mystery. Securities and Exchange Commission Chairman Mary Schapiro said on Tuesday that regulators have thus far failed to find a "single cause" for the nearly 1,000-point Dow Jones Industrial Average plummet. Regulators and exchange officials are scrambling to diagnose the volatility and will be questioned by a House committee looking into the matter. Meanwhile, the Treasury Department has delineated a "structural framework" to bolster market circuit breakers. In written testimony, Schapiro said that the SEC did not find evidence for any "fat finger" trade--otherwise known as the accidental buying or selling of more shares than intended. The media speculated that human error resulting in a fat finger trade caused the meltdown. According to Schapiro, however, "We cannot yet definitely rule that possibility out."