FOR the last two weeks, a justice in New York State Supreme Court has heard testimony in one of the most pivotal cases of the financial crisis. The hearings will tell whether Bank of America can extinguish legal liability for more than a million Countrywide Financial loans by paying $8.5 billion in cash and agreeing to loan servicing improvements in a settlement struck with 22 investors in 2011.

But the case, being heard by Justice Barbara R. Kapnick, extends far beyond the impact of the settlement on Bank of America’s balance sheet. It is also laying bare an industry practice that has put investors in mortgage securities at a disadvantage and reduced their financial recoveries in the aftermath of the home loan mania.

The practice at issue involves trustee banks overseeing the vast and complex mortgage pools bought by pension funds, mutual funds and others. Trustees like Bank of New York Mellon were paid by investors to make sure that the servicers administering these mortgage deals, known as trusts, treated them properly. Trustees receive nominal fees — less than a penny on each dollar of assets — for the work.

But when mortgages soured, trustees declined to pursue available remedies for investors, such as pushing a servicer to buy back loans that did not meet quality standards promised when the securities were sold.