Citibank will pay a huge fine for manipulating an important interest rate.

The bank settled with attorneys general in 42 states for $100 million. Following an investigation, the states said Citibank manipulated Libor, a benchmark interest rate that helps set lending rates across the world.

Citibank made millions of dollars of gains from its "fraudulent conduct," the attorneys general said.

"Our office has zero tolerance for fraudulent or manipulative conduct that undermines our financial markets," New York Attorney General Barbara Underwood said in a press statement. "Financial institutions have a basic responsibility to play by the rules -- and we will continue to hold those accountable who don't."

Related: Société Générale fined $860 million for bribes and rate manipulation

This is the third bank that settled with state attorneys general for illegally influencing the Libor. Barclays, Deutsche Bank and now Citibank have been fined $420 million collectively.

Citibank agreed to comply with ongoing investigations into other banks' Libor cases.