Kirsty Wigglesworth / AP

Barclays Bank has agreed to pay $100 million to 43 states and the District of Columbia to settle allegations it improperly set key interest rates almost a decade ago, affecting payments on investments.

Monday's settlement includes $93.5 million in restitution and costs of the investigation led by the New York and Connecticut attorneys general.

Three more investment banks say they are being investigated in a widening probe by New York Attorney General Eric Schneiderman over allegations the banks give flash traders unfair advantage over regular investors.

Deutsche Bank, UBS, and Credit Suisse have regulators asking questions about who they allow to trade in their private stock exchanges. These so-called "dark pools" have grown rapidly in recent years as slower moving institutional investors seek protection from predatory flash traders.

AP/John Minchillo

Large institutional investors have long been at the mercy of flash traders who use computers to make thousands of trades a second, skimming off tiny bits of profit from pension and retirement accounts. 

For protection fund managers sought out private stock exchanges called dark pools where only certain investors were allowed in. 

Think of them like a fish tank for guppies. 

"What Barclays is accused of is making promises that they would keep sharks out of the pool but were actually sort of inviting the sharks into the pool."