Representative Elijah Cummings, D-Maryland, and Senator Bernie Sanders, I-Vermont, have sent letters to executives of the generic drug makers Mylan, Teva, and Heritage Pharmaceuticals, asking for drug pricing information as part of a probe into the rising cost of generics.

Cummings and Sanders originally sought information on drug price increases in 2014, sending document requests to 14 generic drug makers. Among those companies, the 3 who received letters from the lawmakers this week did not produce documents, despite what Cummings and Sanders say were repeated inquiries and discussions with the companies’ legal teams.

Cummings’ and Sanders’ letters come on the heels of the May 10 filing of a complaint by 44 US states that alleges that manufacturers coordinated their efforts to inflate the prices of several drugs covered in the 2014 probe.

Connecticut’s Attorney General, William Tong, recently released the full and unredacted complaint against drug makers including Teva, Mylan, Pfizer, Sandoz, and others. According to the complaint, the generic drug industry “would systematically and routinely communicate with one another directly, divvy up customers to create an artificial equilibrium in the market, and then maintain anticompetitively high prices,” and the drug makers “embarked on one of the most egregious and damaging price-fixing conspiracies in the history of the United States.”

The complaint also alleges that Mylan, Teva, and Heritage coordinated to obstruct Cummings’ and Sanders’ 2014 investigation into price increases. In their letters, Cummings and Sanders warn the drug makers that obstructing or evading a Congressional investigation is in violation of federal law.

Cummings and Sanders are now asking the companies for documents including those detailing revenues, expenses, sales contracts, and profit projections for specific drugs, as well as documents that would identify phone calls between employees of Heritage, Mylan, and Teva. The lawmakers have given the companies until August 28 to respond to their request.