Leading Democrats want to know if drug maker Mylan avoided for decades paying a higher Medicaid rebate for the EpiPen, whose $600 price tag has infuriated lawmakers.

Rep. Frank Pallone, D-N.J., and Sen. Ron Wyden, D-Ore., wrote to Health and Human Services Secretary Sylvia Burwell Friday asking about the status of the rebate for Mylan’s EpiPen. The letter is the latest inquiry from Congress into a startling 400 percent price hike for the life-saving allergy treatment.