The federal commission probing the financial crisis took aim at the causes of the subprime mortgage meltdown, but former Federal Reserve Chairman Alan Greenspan said Wednesday that it shouldn't point at him.

Summoned to address sharp criticism that the Fed failed to stop the housing bubble and the risky mortgages that helped pop it, Greenspan strongly defended his actions and warned that regulators alone couldn't stop financial crises.

The best prevention, he said, would come from increasing requirements on banks and other financial institutions to have more money and collateral to carry them through rough times.

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