U.S. authorities have called it one of the biggest cases of corporate collusion in history, costing patients billions of dollars.

In September 2016, the FBI raided the suburban Pittsburgh headquarters of generic drug giant Mylan NV. Three years later, a sweeping federal investigation into the generics industry has hit significant obstacles — even as more than 40 state attorneys general have filed their own state-level civil complaints with specific claims of collusion.

Those lawsuits detail extensive telephone contacts and meetings among executives at competing generics makers that preceded allegedly coordinated price hikes on drugs.