NEW YORK (CNNMoney) -- Lawmakers scolded the FDA for still not knowing what or who was behind the contamination of the blood-thinning drug Heparin, nearly three years after launching its investigation.
The contaminated drug led to the deaths of dozens of people in the U.S.
"It has been almost three years since the the FDA linked deaths and serious allergic reactions of patients to supplies of Heparin that came from China," Rep. Fred Upton (R-Mich.), wrote in a letter to Food and Drug Agency Commissioner Margaret Hamburg Wednesday.
Upton said the House Energy and Commerce committee, which he chairs, will conduct its own probe into the matter.
Neither the Chinese government nor the FDA has identified those responsible, even though FDA officials believe the contamination, with overly sulfated chondroitin sulfate, was economically motivated, according to the letter.
Upton said, "some of the individuals responsible for the adulteration are actively engaged in the Chinese pharmaceutical supply chain and pose a continuing threat to pharmaceutical products imported to the U.S."
Heparin is administered to about 12 million people in the U.S. annually, the committee said.
The agency has two weeks to submit all documents related to Heparin-related inspections conducted in China and possible sources of adulteration.
FDA spokeswoman Karen Riley said the agency will review the letter and respond directly to lawmakers.
FDA under the gun: Lawmakers are increasingly questioning why so many problems with drug manufacturing quality have gotten by safety regulators.
The agency announced an investigation -- including a criminal probe -- into those recalls last summer but has not yet issued its findings.
While many drugs are manufactured in the U.S., a growing number are coming from China, which poses a challenge to regulators.
80% of all active pharmaceutical ingredients used in drugs sold in the U.S. now originate overseas, according to the U.S. Government Accountability Office. China is the top supplier of those ingredients.
The FDA typically inspects drug manufacturing plants in the U.S. every two years.
But with 1,000 or more drugmaking plants in China supplying U.S. companies, not every facility can be inspected on a three-year schedule, said Allan Coukell, director of medical safety with research firm Pew Health Group.
The FDA conducted 52 inspections in China in 2009, said agency spokeswoman Riley.
A group of Democratic lawmakers is calling on a government agency to determine whether President Donald Trump has violated a term of the lease for his hotel in Washington, D.C. More
Donald Trump promised to call China out for being a 'currency manipulator' on day one. It didn't happen. CNNMoney explains what exactly that means and whether it is likely to happen later. More
In 1998, Ntsiki Biyela won a scholarship to study wine making. Now she's about to launch her own brand. More
The CFPB has fined CitiFinancial Servicing and CitiMortgage $28.8 million for giving 'the runaround' to cash-strapped homeowners who were trying to save their homes. More