It may be medically impossible to prove that deadly doses of insulin were administered to eight elderly patients who police believe were murdered by a nurse.

According to criminal defence lawyer Christopher Hicks, insulin remains in the body for only about two days.

"Insulin is a very volatile substance,” Hicks told CTV News. “It's gone in 48 hours. No pathologist will find a trace of it past that time."

Elizabeth Wettlaufer, 49, of Woodstock, Ont., is suspected of administering deadly doses of a drug to patients at two nursing homes. The victims were between the ages of 75 and 96 and died from 2007 to 2014 in Woodstock and London, Ont.

Sources have told CTV News that the suspected drug was insulin.

CTV News has also learned that Wettlaufer allegedly confessed to the killings while undergoing addiction treatment for narcotics at Toronto’s Centre for Addiction and Mental Health. Sources say officials from CAMH then reached out to authorities to report the alleged confession. Ontario health care providers are required to contact police in situations where a homicide is suspected.

A former nurse at Caressant Care Long Term Care Home in Woodstock, where seven of the eight victims were allegedly killed, said she isn’t surprised by the allegations of insulin overdoses.

"There's no way it would've been detected,” said retired nurse Rosalynd Sim.

Ontario Provincial Police have said they have no plans to exhume the bodies of the victims. Police have not identified the alleged drug in question.

Meanwhile, an anonymous letter sent to CTV News claims that “a lot of med errors" were reported regarding insulin at the Woodstock facility but “nothing done.” The hand-written letter states that Wettlaufer “had a long list of disciplines” at the nursing home and that she was told by administration “if she went quietly they would not call the college (of nurses).”

CTV News has not independently verified the letter’s claims.

Representatives from the Caressant Care Long Term Care Home declined to comment to CTV News, saying that the letter was anonymous and its claims unsubstantiated.

It’s unclear if there were any investigations into the deaths of the eight seniors. The Ontario Coroner’s Office has not commented on the matter.

The daughter of Arpad Horvath, the sole victim at London’s Meadow Park Long Term Care facility, said she demanded an investigation into her father’s death in 2011 after he suddenly fell into a coma. She said her father’s abrupt change in health was “eerie” and she suspected he was drugged.

Despite her pleas, she says there was no investigation.

"He didn't have an autopsy and there's no toxicology reports," Susan Horvath said.

Insulin is commonly used to treat patients with diabetes. But too much insulin can lead to hypoglycemia, a coma and, in extreme cases, death.

In the 1990s, Ontario introduced a requirement to investigate every tenth death at nursing homes in the province. In 2013, that policy was scrapped to only cover cases where abuse or wrongdoing was suspected.

Ontario’s health minister stood behind the current policy when asked about it Thursday.

"The coroner has expressed confidence in the system that's in place,” said Ontario Health Minister Eric Hoskins.

Wettlaufer is scheduled to appear in court by video on Nov. 2.

With a report from CTV’s Peter Akman