Four people who worked at Valeant Pharmaceuticals International Inc. have been asked by Democrats to come to Washington and answer questions about the company’s relationship with a mail-order pharmacy.

U.S. Representative Elijah Cummings, a Maryland Democrat, wrote Valeant Chief Executive Officer Michael Pearson on Monday requesting he make the employees available for transcribed interviews. Workers at the pharmacy, Philidor Rx Services, sometimes changed codes on doctors’ prescriptions to ensure they were filled with the brand name instead of the generic, according to the former employees. Philidor said that it only filled prescriptions with medications that doctors and patients requested, but Valeant has said it’s cutting ties with the pharmacy.

Cummings wants to interview Gary Tanner, Bijal Patel and Alison Pritchett based on allegations “that a group of Valeant employees helped launch Philidor’s business in 2013 and have remained involved in its daily operations,” according to the letter. Cummings also asked for contact information for former employee Laizer Kornwasser. Tanner left Valeant in September and joined Philidor, Bloomberg reported earlier this month.

The letter is part of Cummings’s ongoing investigation into Valeant and Turing Pharmaceuticals AG for acquiring older drugs and then significantly raising their prices. Drug pricing has received intense scrutiny in Washington lately. The U.S. Senate’s Special Committee on Aging plans to hold a hearing next month on drug-pricing practices by Valeant, Turing and two other pharmaceutical companies.

Cummings, who is the top Democrat on the House Oversight and Government Reform Committee, has tried to get Committee Chairman Jason Chaffetz to hold a hearing as well as subpoena Valeant and Turing to compel them to hand over information on price increases. Cummings wrote Chaffetz earlier this month asking for a panel vote Tuesday to subpoena Pearson and Turing Chief Executive Martin Shkreli.

MJ Henshaw, a spokeswoman for Chaffetz, didn’t immediately respond to a request seeking comment on whether the committee would vote on the subopoenas.

Valeant raised the prices of two heart drugs, Nitropress and Isuprel, by 212 percent and 525 percent the day the drugmaker acquired the rights to sell them. Closely held Turing bought a decades-old anti-parasitic treatment and increased the price to $750 a pill from $13.50.