Moody's Investors Service has downgraded $2.2 billion of securities backed by Alt-A mortgages issued in 2005 by Residential Funding Co., a unit of what is now Ally Financial.

According to wire service reports, Moody's has downgraded several hundred billion dollars of MBS since the start of April as falling home prices, high joblessness and the slow economy led credit raters to revise loss expectations.

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