Allstate Corp. has filed a federal lawsuit against Countrywide Financial Corp. over $700 million in toxic mortgage-backed securities that the insurer bought beginning in 2005, only to see their value decline rapidly (See Complaint and Exhibits in the case).

The suit will test whether Allstate’s drastic losses were the result of an unanticipated decline in the housing market or, as it claims, Countrywide’s “wrongdoing.”