Chinese Food and Drug regulators said pharmaceutical Baxter International Inc. did not fully co-operate in an investigation to address the case of the blood thinner heparin to which 81 deaths are linked due to the presence of a chemical contaminant.

China's State Food and Drug Administration said the company didn't provide requested samples and information on the manufacturing of heparin to Chinese investigators at the time they visited a plant of Baxter in New Jersey on April 22.

Heparin's key ingredient is made from pig intestines much of which comes from unregulated small workshops and farms in China. The treatment was recalled by Baxter when a chemical, oversulfated chondroitin sulfate, was found to potentially have killed 81 patients and cause allergic reactions.

United States regulators speculate the contamination was made intentionally since the chemical is similar to the main active ingredient but cheaper.

But Chinese officials say that the chemical was not the root cause of deaths or side effects and it still investigating how the chemical was added to the heparin.

Apart from the U.S. and Germany, more than 10 other countries using heparin products containing the 'heparin-like substance' have not reported adverse reactions, the SFDA said in a statement.

Jin Shaohong, of China's National Institute for the Control of Pharmaceutical and Biological Products, said Baxter had also destroyed some samples of Heparin and production records that China considered significant to follow the investigation and reasons for the problems.

I was very surprised, said Jin. This is extremely regrettable.

Baxter replied saying the firm has been and will continue co-operating to advance the investigation with all parties in the heparin situation including SFDA and the Chinese government. We will seek to understand any concerns to the contrary, the firm noted.

Shares of Baxter International closed Wednesday composite trading 1.99 percent lower to $61.47 on the New York Stock Exchange. Shares rose to $62.03 in after-hours electronic trading.