China and the United States are working together to investigate the blood-thinner heparin, which has been linked to 19 American deaths, China's food and drug administration said Sunday. The U.S. Food and Drug Administration has been widening its investigation into the hundreds of adverse reactions - including difficulty breathing, nausea and falling blood pressure - linked to U.S. health care company Baxter International's heparin injections. Heparin is derived from pig intestines, and China is the world's leading supplier.

U.S. and Chinese officials have been investigating heparin samples but have reached no conclusions, Wu Zhen, the deputy commissioner of China's State Food and Drug Administration, said Sunday.

"China's State Food and Drug Administration and America's FDA are working closely together on the heparin investigation, and American investigators came to China to investigate and we supported them," Wu said at a news conference.

"The most important thing now is that the scientists of both sides launch an in-depth investigation into the details," he said. "We can't take effective measures until we find the cause."

The U.S. FDA found a contaminant in 20 of 28 samples of raw heparin from Baxter's main supplier, Scientific Protein Laboratories of the United States, which gets the bulk of its heparin from a joint venture in China's east Jiangsu province.

Scientists don't yet know exactly what the contaminant is, except that it mimics heparin so closely that standard drug-purity tests will not catch it. Nor is it certain that the contaminant is to blame for the allergic reactions, although it is the prime suspect.

Wu said it was still unclear at what stage the heparin had been contaminated because the production process was long and complicated. He said Scientific Protein Laboratories got its raw material from Kaipu Biochemical Co. in Changzhou, also known as Changzhou SPL.

Wu said the heparin could have been contaminated after production or when the drug was exported overseas.

A different brand of heparin has also been recalled in Germany after 80 patients there became sick, and the German manufacturer said it was narrowing down the source of contamination to another Chinese supplier.

The mayor of Changzhou, Wang Weicheng, said Friday that the city government had cooperated with the investigation and was seriously monitoring the companies in the city. He said it was the first time a problem had been reported.

The city shut down 800 chemical companies last year for violating environmental standards, he said.