Former Federal Reserve Chairman Alan Greenspan swung back at his critics Wednesday by pointing out that Congress pushed the U.S. central bank to make sure lending to poorer Americans kept rising in the 2000s.

"If the Fed as a regulator had tried to thwart what everyone perceived as a fairly broad consensus that the trend was in the right direction, homeownership was rising and that was an unmitigated good, then Congress would have clamped down on us," he told a questioner at a congressionally appointed commission investigating the financial crisis.

"There's a presumption that the Federal Reserve's an independent agency, and it is up to a point, but we are a creature of the Congress and if ... we had said we're running into a bubble and we need to retrench, the Congress would say 'We haven't a clue what you're talking about,'" Greenspan said.

Greenspan told the committee that making it easier for poorer Americans to get mortgages didn't push the country into crisis, but Wall Street's drive to package the loans into opaque securities helped do so.

He renewed a defense of his own legacy by denying the U.S. central bank helped inflate housing prices with excessively low interest rates.

"The house price bubble, the most prominent global bubble in generations, was caused by lower interest rates but ... it was long-term mortgage rates that galvanized prices, not the overnight rates of central banks, as has become the seeming conventional wisdom," Greenspan said.

Much of Greenspan's testimony retraced the ground that he trod in a scholarly defense of Fed policy in the early 2000s three weeks ago at the Brookings Institution.

Many now blame prolonged low official rates from 2001 to 2003 for helping set U.S. house prices on fire and contributing to a slackening in standards for loans by banks and rating agencies.

"Let me respectfully restate that, in my judgment, the origination of subprime mortgages — as opposed to the rise in global demand for securitized subprime mortgage interests — was not a significant cause of the financial crisis," Greenspan said.

He repeated that geopolitical events including the fall of the Soviet Union and the rise of China helped boost the global workforce, pushing up growth and creating a global savings glut that drove down long-term interest rates.

Greenspan, who led the Fed from 1987 to 2006, conceded that regulators and others miscalculated the risks that were involved in a buildup of U.S. housing prices in the 2000s.

"In the growing state of euphoria, managers of financial institutions, along with regulators including but not limited to the Federal Reserve, failed to comprehend the underlying size, length and potential impact" of market risks, he said.

Rating agencies that assigned risk ratings to securities underlying rising mortgages issuance "proved no more adept at anticipating the onset of crisis than the investment community at large," Greenspan added.

He said a proliferation of securitized U.S. subprime mortgages was the "immediate trigger" of the crisis and said government-owned enterprises Fannie Mae and Freddie Mac helped drive that process.

Greenspan said that the key question for regulators seeking to avert future financial crises is to decide what capital levels financial firms should meet.

Looking ahead, he recommended that U.S. banks be compelled to meet higher standards for risk-based capital and that collateral requirements be boosted for globally traded financial products, no matter what firm is trading them.

"If capital is adequate, by definition, no debt will default and serial contagion will be thwarted," he said.

"You are raising exactly what the appropriate issue is that should confront regulators: What is adequate capital?" He added that he had definite views on the matter but didn't specify what percentage level of capital he felt banks and others should be required to keep on hand.

Greenspan also said that banks, and possibly all financial firms, should be required to hold contingent capital bonds that convert to equity if capital fell below set levels.

That would reduce the problem of so-called "moral hazard" by reducing a perception that the government would step in and bail them out in times of trouble.