WASHINGTON, Nov. 4 – Sen. Bernie Sanders (I-Vt.) and Rep. Elijah Cummings (D-Md.) sent a letter Thursday asking the Justice Department and the Federal Trade Commission to investigate potential collusion among pharmaceutical companies that manufacture diabetes products.

The prices of insulin medications, which are used by about 6 million Americans to treat diabetes, have skyrocketed in recent years. The cost of insulin more than tripled between 2002 and 2013, from $231 to $736 per year per patient.

“Not only have these pharmaceutical companies raised insulin prices significantly – sometimes by double digits overnight – in many instances the prices have increased in tandem,” Sanders and Cummings wrote in the letter to Attorney General Loretta Lynch and Federal Trade Commission Chairwoman Edith Ramirez. “We are concerned that the potential coordination by these drug makers may not simply be a case of ‘shadow pricing’ but may indicate possible collusion.”

Three diabetes drug makers have continuously raised their prices over the years, and in numerous instances the price increases have mirrored one another precisely. In May 2014, Sanofi and Novo Nordisk each raised their prices for a vial of diabetes medication by 16.1 percent within a day of each other. Six months later, the companies both raised the medication price by 11.9 percent.  There have been 13 reported instances of such tandem price increases between those two companies since 2009.

Eli Lilly has also on several occasions matched price increases of its insulin products with Novo Nordisk. Eli Lilly and other companies have previously been fined in Mexico for colluding on insulin pricing.

“We believe this egregious behavior warrants a thorough investigation,” the members wrote.

To read the letter, click here.