To plaintiffs firms, there are financial scandals, and then there is Libor.

Libor, the London interbank offered rate, is a benchmark interest rate that serves as a foundation for trillions of dollars of financial products worldwide. Suspicions that banks involved in setting the rate were manipulating it spurred investors to file lawsuits beginning in 2011. Now, a growing number of plaintiffs firms are joining the fray in light of news this summer that at least one bank, Barclays Bank PLC, admitted to misconduct.