WASHINGTON — The investigation into a blood thinner suspected in 19 U.S. deaths is focusing on the possibility that raw biological ingredients were contaminated even before they reached a factory in China, manufacturer Baxter Healthcare Corp. said Friday.

That raises the prospect that the problem could have occurred somewhere along a supply chain that includes layers of middlemen and originates in pig farms. The generic medication, heparin, is derived from a substance found in the lining of pig intestines. It is used widely to prevent dangerous blood clots from forming during kidney dialysis and cardiac surgery.

The case has reinforced concerns about quality-control problems with Chinese products. But unlike last year's pet food recall, it was frail patients who suffered the ill-effects of the drug contamination. Some suffered life-threatening allergic reactions.

The Food and Drug Administration announced new testing requirements for all imported heparin Friday. Manufacturers that do not agree in advance to sophisticated safety tests will have their shipments held at ports of entry until they can be tested by the government.