Authorities in India say they are trying to determine whether contaminated antibiotic pills were the cause of the death of 13 women who had undergone tubal ligation in a government-run "sterilization camp."

After their surgeries, the women were given pills of the antibiotic drug ciprofloxacin manufactured by Mahawar Pharma, a pharmaceutical company supplying drugs to the government of the state of Chhattisgarh, one of the poorest in India.

A chemical known as zinc phosphide was detected in the tablets, a senior official said.

"If you do a quick Google search, you will find it is rat poison, and the women were displaying symptoms similar to poisoning," said Sonmoni Borah, the divisional commissioner in the state's Bilaspur district.

Authorities discovered stocks of zinc phosphide at Mahawar Pharma and suggest it "was mixed during manufacturing," he said.

However, a number of medical experts said it was doubtful zinc phosphide, even it if was present in the antibiotic tablets, would have been fatal.

Symptoms of zinc phosphide poisoning include abdominal pain, nausea and vomiting, and while that matched what some of the women had reported, a 110-pound woman would have to ingest a minimum of two full grams of pure zinc phosphide to run the risk of dying, experts said.

Some of the symptoms are also commonly seen in septic shock, always a possibility in any surgical procedure such as the tubal ligation the women underwent, they said.

In fact, a preliminary autopsy on one of the patients who died suggested septicemia as the cause, officials said.

One of the doctors conducting autopsies said the results were not conclusive and recommended further chemical analysis.

Adulterated and counterfeit medicine constitutes a significant problem in India. The World Health Organization (WHO) estimates one in five pharmaceutical drugs manufactured in the country is likely to be fake.

Episodes of poisoning are fairly rare, however, because most of the counterfeit drugs contain little active ingredients or often none at all.

Most Indian states are without the personnel or budget to perform extensive drug inspections, leading to weak oversight of the drugs they procure.

"States procure medicine through a tender and the manufacturers that bid the lowest quote win the order to supply, regardless of their manufacturing process or distribution systems," says Bejon Kumar Misra, head of the non-governmental organization Partnership for Safe Medicines India.

India is a major exporter of prescription and over-the-counter medicine around the world, including the United States, a cause for concern among regulators in the U.S.

Safety concerns have already led the U.S. Food and Drug Administration to ban the importing of Indian-made generic ciprofloxacin.