Standard & Poor's lowered ratings for 224 classes of residential mortgage-backed securities after finding a system error. Uncovering the error, which analysts said "caused the loan-group mapping for these transactions to be incorrectly displayed in our surveillance systems," led to the review of ratings of 470 classes of various types of RMBS issued across 50 transactions between 1997 and 2007. The ratings of 17 classes of RMBS were upgraded following the review. Most of the affected RBMS were sold in deals led by American Home Mortgage Investment Trust; GSR Mortgage Loan Trust; Lehman Mortgage Trust; Citigroup Mortgage Loan Trust; Structured Adjustable Rate Mortgage Loan Trust; Structured Asset Securities Corporation Trust; and WaMu Mortgage Pass-Through Certificates. Bloomberg reported Thursday afternoon that Standard & Poor's has downgraded $2.3 trillion of the $3.3 trillion of RMBS issued between 2005 and 2007 alone. Also Thursday, Moody's Investors Service said it is reviewing billions of dollars of commercial mortgage-backed securities for possible downgrades to account for increased losses from specially services and troubled loans. The ratings agencies have come under increasing scrutiny as the mortgage crises unfolds with some politicians, investors and pundits claiming analysts rated mortgage bonds improperly, according to Bloomberg. Write to Jason Philyaw.