Only seven people in the US died of cyanide poisoning in 2012; almost as many were killed by lightning this year in Florida alone. But at one medical center in Pittsburgh, two women have died of it in two years, both in mysterious circumstances.

On 17 April 2013, Dr Robert Ferrante called 911 to report that his wife, Dr Autumn Klein, 41, had suddenly taken ill. Three days later, at the University of Pittsburgh Medical Center, the hospital where she worked as chief of women’s neurology, she died.

Klein’s case initially stumped her colleagues who were treating her. It was only when they spread the search for the cause of her illness as wide as they could that they discovered what had gone wrong. In the course of performing several obscure tests they found the cyanide that would go on to kill her. They “ordered everything including the kitchen sink,” the Allegheny County chief medical examiner, Dr Karl E Williams, told the Guardian, “and the cyanide test was part of that kitchen sink.”

Investigators looking into how Klein could possibly have been killed in such an unusual way discovered that Ferrante had ordered a large overnight shipment of cyanide to his lab two days before she became sick. Ferrante, prosecutors would later say, gave Klein a creatine drink, saying it would help them conceive a sibling for their then six-year-old daughter – but that before he gave it to her, he laced it with a lethal dose of the poison.

On 7 November this year, Ferrante was convicted of his wife’s murder. Five days later, molecular biologist Nicole Kotchey, 34, was found collapsed near her desk at UPMC. Less than four hours later, she died. An autopsy conducted later showed that Kotchey had become the second woman at UPMC to die of cyanide poisoning.

“To have one – let alone two – is extraordinarily unusual,” Williams said. His office worked on both cases and last month announced that Kotchey’s death was caused by cyanide. The exact manner of death is yet to be determined.

Gregg Kotchey, Nicole’s brother, has asked that the family be given privacy during the investigation. He celebrated his sister in a statement, calling her a “kind-hearted and loving daughter and sister”.

“The family is still grieving over the loss of Nicole, which is especially hard during the holiday season”, Kotchey said.

The medical examiner’s office is working with police to determine whether the manner of death is homicide, suicide or an accident – though the medical examiner also has the power to rule it “undetermined”.

Williams said there has been no clear indication of which of the three it could be. He also would not say how Kotchey was exposed to the cyanide. Lethal amounts can be ingested, absorbed through the skin or inhaled in gas form – though that happens almost exclusively in industrial settings.

Investigators have not yet found any history of depression in Nicole Kotchey’s background, nor any indication of suicidal behavior. Williams said her death is unlikely to be homicide.

Kotchey is listed on two research papers posted online: one from 2011, the other from 2008. The studies examine viral vectors, a tool for transferring genes to modify cell types or tissue. These papers look at how these processes can be used to repair genetic errors for things like neonatal gene therapy. It is not clear whether she worked with cyanide for her research.

“It’s just very strange because if you do the kind of research that involves cyanide, you are obviously very aware of its risks,” said Nathan Lents, associate professor of molecular biology at John Jay College of Criminal Justice.

He said investigators are probably examining every part of her life to find someone who might be interested in killing her. He also noted that cyanide purchases are scrutinized. “You can’t just open a catalog and buy it,” said Lents.

In the US, seven people died from cyanide poisoning in 2012, according to the most recent annual report by the American Association of Poison Control Centers (pdf). Five of the deaths are classified as suicides, one is unknown and the other is unintentional poisoning from a drug that opens blood vessels and contains a cyanide compound.

The lethal chemical is better known for its use by the Nazis and in the 1978 mass cult suicide-murder in Jonestown, Guyana. More than 900 people died there, including hundreds of children, most from consuming cyanide-laced punch.

Williams said he did not know why his office performed the obscure test for the chemical on Kotchey, but it was probably prompted by the results of the Klein case.