WASHINGTON, March 14 (UPI) -- The U.S. Food and Drug Administration has issued an import alert and plans to test all shipments of the drug heparin before they enter the country.

Multiple batches of heparin sold by Baxter International have been been found to contain a mysterious contaminant, The Wall Street Journal reported Friday. The contaminated product was made in a Chinese plant.

Baxter told the newspaper company officials think the contaminant comes from pig intestines used to produce the popular anti-coagulant. The company said four recent deaths may have been at least partly related to reactions to its heparin.

Five manufacturers have agreed to test all shipments of heparin and its main ingredients for contamination, the newspaper said.