A pair of U.S. senators called on the federal government to investigate health insurers that are paying high prices for generic drugs for serious diseases like cancer and multiple sclerosis.

Sen. Elizabeth Warren (D., Mass.) and Sen. Mike Braun (R., Ind.) sent a letter on Wednesday to the U.S. Department of Health and Human Services’ Office of Inspector General requesting an investigation into the high drug prices and any role played by health insurers’ shared ownership with the pharmacies that often fill the prescriptions.