The New York attorney general has begun an inquiry into Turing Pharmaceuticals, whose fiftyfold overnight increase in the price of a 62-year-old infection drug stoked a backlash against high pharmaceutical prices.

The attorney general’s office is looking not so much at the price increase itself but at whether Turing may have violated antitrust rules by restricting distribution of the drug, Daraprim, as a way to thwart generic competition, according to a letter sent by the attorney general’s office to Turing on Monday.

“While competition might ordinarily be expected to deter such a massive price increase, it appears that Turing may have taken steps to prevent that competition from arising,” said the letter, a copy of which was obtained by The New York Times.

The letter was addressed to Martin Shkreli, the 32-year-old former hedge fund manager who runs Turing, and was signed by Eric J. Stock, the chief of the antitrust bureau. It asks that Turing contact the attorney general’s office immediately to discuss the situation and tells the company to “retain all documents that are potentially relevant to this inquiry.”