Attorneys general in 50 states are investigating the industry’s use of mortgage assignments as part of a wider probe into faulty foreclosure methods, according to Geoff Greenwood, a spokesman for Iowa attorney general Tom Miller. Last month, a Massachusetts court ruled that two foreclosures by Wells Fargo & Co. and U.S. Bancorp were invalid because assignments presented in those cases failed to prove the chain of ownership of the mortgage, sending financial stocks down.