The booming market for commercial mortgage-backed securities is on the verge of passing an uncertain milestone: losses on a class of bonds that have been popular with investors for the big returns they promised. 

"AJ" securities have found favor with investors because they combine high payouts and greater protection against losses than some other high-yielding securities. 

But now, the potential sale of a troubled Atlantic City luxury mall named the Pier Shops at Caesars could saddle investors with the first losses on AJ securities created during the U.S. housing boom. The property is on the block after an earlier sale fell through.