Health officials investigating the deaths of more than 13 women who died after attending government-run family planning camps in eastern India believe that they were killed by medicine contaminated with rat poison.

All antibiotics bought from Mahawar Pharmaceuticals, a factory in the the eastern city of Raipur, the state capital, have now been withdrawn, the Press Trust of India reported.

Quantities of zinc phosphide, a component of rat poison, was found at the factory, where antibiotics distributed at the two camps on Saturday and Monday were made earlier this week.

“We have seen some toxin reaction in some patients, not infection. Renal failure, falling blood pressure, in all cases issued with this drug. What they were given was not antibiotic but a toxin,” said Dr Ashutosh Tiwari, secretary of the local branch of the Indian Medical Association in Bilaspur, where the deaths have occurred.

The results of tests on pills given to women and on the viscera of the casualties are still awaited, for final confirmation of the presence of the poison in the drugs.

Around 20 women who attended the camp remain in intensive care and the death toll is expected to rise.

More possible victims of the contaminated antibiotics arrived at hospitals from villages in Bilaspur district, about 60 miles (100km) from Raipur, on Thursday and Friday, some clutching medicine strips from Mahawar and complaining of vomiting, dizziness and swelling.

At least one of the strips of antibiotic was from the same batch as those handed out at the mass sterilisation held on Saturday in the same district in Chhattisgarh state, one of India’s poorest.

Tiwari said two other patients, both male, had died of total renal shutdown after taking antibiotics from the same batch. They had been prescribed the drugs by local practitioners for minor problems.

“There was something in it which we use to kill rats in India. It is this that caused the mortality,” Tiwari, who has been briefed on the results of the ongoing official inquiry, told the Guardian.

Police say they entered the Mahawar factory on Wednesday with the help of a security guard, but at first found nothing wrong. Drug inspectors returned the next day and shut it down.

Speaking in police custody, Ramesh Mahawar, managing director of Mahawar Pharmaceuticals, said he was innocent.

“The situation has been twisted in a wrong manner. We are just being harassed,” said Mahawar, who has been making drugs for 35 years.

The focus has now shifted away from the appalling conditions at the camp where veteran gynaecologist Dr R K Gupta conducted one tubectomy every two minutes in a two-hour sterilisation drive in a dirty room in a disused hospital.

The deaths in Chhattisgarh have drawn attention to India’s mass-sterilisation programme, as well as weak quality-control standards for drugs procured by state governments.

“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,” said Bejon Kumar Misra, head of Partnership for Safe Medicines India, a non-governmental organisation.

But G N Singh, the drugs controller general of India, said quality and safety came before price in the tender process.

“If the drugs are found to be substandard, we will suspend the licence of the manufacturer,” he said.

Corruption is rife in a sector long seen as a problem by health experts and campaigners.

India is the world’s top steriliser of women, and efforts to rein in population growth have been described as the most draconian after China. Indian birth rates fell in recent decades, but population growth is still among the world’s fastest.

Sterilisation is popular because it is cheap and effective, and sidesteps cultural resistance and problems with distribution of other types of contraception in rural areas.

Deaths due to sterilisation are not new in India, where more than 4 million of the operations were performed in 2013-14, according to the government.

Between 2009 and 2012 the government paid compensation for 568 deaths resulting from sterilisation, the health ministry said in an answer to a parliamentary question two years ago. A total of 1,434 people died from such procedures in India between 2003 and 2012.

Experts said on Thursday the true figure was much higher.

“Not all are reported, not all get compensation. Then there are those who suffer complications or side-effects, often due to infection. There is no data on them at all,” said Sonam Sharma, at the New Delhi-based Population Foundation of India.