U.S. Democratic presidential candidate Sanders pauses during a speech at a campaign event in Chicago Thomson Reuters US Sen. Bernie Sanders (I-VT) gave Turing Pharmaceuticals three weeks to respond to a letter requesting more information about its controversial recent price hike on the critical drug Daraprim.

Sanders and co-author Rep. Elijah Cummings (D-MD) are investigating sudden hikes in the costs of older medicines. In their letter to Turing, the pair requested that the company hand over information about the company and its revenues for the sale of the drugs as well as the identities of individuals in the company responsible for the recent price hike, which raised Daraprim's sticker price from $13.50 to $750 a pill.

The 62-year-old drug is commonly used to treat patients with AIDS, cancer, and malaria.

Sanders and Cummings have been putting pressure on drug companies for a while, reaching out to them to explain drug price hikes and considering legislation to curb pharmaceutical drug prices in general.

The deadline for responding to their letter — Friday, October 9 — has come and gone, and Sanders and Cummings have yet to get a response from Turing. And as of last week, the price of the drug had not been reduced.

Sanders and Cummings are not happy.

“On behalf of the American people, we are sickened by these actions," Sanders and Cummings said in a news release Friday. "Mr. Shkreli is holding hostage the patients who rely on this lifesaving medication, as well as the hospitals that administer it, by charging unconscionable prices for a drug on which he has a monopoly — just because he can."

In the meantime, Turing has hired lobbyists, perhaps to fend off the requests of the letter. Shkreli, who set his Twitter account to private after announcing that he would lower the price, has now returned to public Twitter in full force, responding to critics and announcing company news.