“WHAT did they know, and when did they know it?” Those are questions investigators invariably ask when trying to determine who’s responsible for an offense or a misdeed.

But for the Wall Street banks whose financing of the subprime mortgage machine placed them at the center of the credit crisis, it’s becoming clear that a third, equally important question must be asked: “What did they do once they knew what they knew?”

As investigators delve deeper into the mortgage mess, they are finding in too many cases that Wall Street firms did nothing when they learned about problem loans or improprieties in lending. Rather than stopping practices of profligate originators like New Century, Fremont and Ameriquest, Wall Street financiers, which held the purse strings for these companies, apparently decided to simply look the other way.

Recent cases have provided glimpses of this conduct. Last week, the Financial Industry Regulatory Authority accused Deutsche Bank Securities, a unit of the huge German bank, of misleading investors about how many delinquent loans went into six mortgage securities worth $2.2 billion that the firm underwrote. Deutsche Bank underreported the delinquency rates among loans when it created the securities in 2006, Finra contends, and then sold them to investors.