A family in India who died after eating watermelon were actually killed by rat poison, forensic investigators said on Friday, two weeks after the deaths caused widespread concern about the possible dangers of eating the fruit.

Authorities in the western Indian state of Maharashtra are yet to decide whether to register the case as homicide or an accident, and have not disclosed how the watermelon came to be laced with poison.

In the early hours of 27 April, Abdullah Dokadia, 45, his wife Nasreen, 35, and their daughters Ayesha, 16, and Zainab, 13, died at their home in Pydhonie in south Mumbai’s Bhendi Bazaar area.

The family had hosted nine guests for dinner that evening and served biryani. After the relatives left, the four ate watermelon at around 1am. By 5am all were severely ill with vomiting, stomach pain and diarrhoea. They were taken to a local hospital in a semi-conscious state, police said, and all died soon after.

As news of the tragedy spread, it sparked a panic and sent watermelon prices in the area crashing.

None of the family’s dinner guests, who had eaten the same biryani earlier in the evening, fell sick.

Two weeks on, forensics investigators have confirmed the presence of zinc phosphite, a toxic chemical used in rat poison, in both the fruit consumed by the victims and their internal organs, including spleen, liver, kidneys, and stomach.

Police said that no other food item served that evening tested positive for zinc phosphide. “There was no indication of external contamination on the fruit. The poison was on the inside. It’s unlikely to have entered the fruit through an injection,” an unnamed forensics official told The Times of India. “The family had vomited out a large portion of the toxin, which is why it required 54 rounds of testing to identify the trace amounts remaining,” they added.

Officials noted that there was no trace of the toxin inside the Dokadia house.

Police are now investigating how the toxin ended up inside the fruit.

“The report will be studied in depth,” deputy police commissioner Pravin Mundhe said. “In consultation with forensic doctors, further investigation will be continued.”

Police had initially registered a case of accidental death and recorded the statements of the family’s dinner guests.

In the wake of the deaths, local media reported that demand for watermelon in the region had fallen by almost 30 per cent. Traders were offloading the popular summer fruit for as little as Rs 5-7 (4-5p) a kilo, a fraction of its usual price. Retail prices for watermelon in Mumbai typically range between Rs 30 and Rs 100 (23p to 78p) per kilo.

The Maharashtra government had earlier said there was no direct link between the deaths and watermelons and that no conclusions would be drawn until scientific confirmation was received.