BUFFALO, Minn. (FOX 9) - It was a medical mystery that turned into a murder investigation.

‘Devastation all over again’

What we know:

It's hard enough for Nickole O'Neill to know her daughter's father has passed away at the age of 33.

But learning he was intentionally poisoned is ever tougher to take.

"I knew somebody did something, but I didn't expect it to be so close," said O'Neill.

‘He wanted Cody to die’

The backstory:

Last week, prosecutors charged Stuart Hanmer with second-degree murder for allegedly poisoning his roommate, Cody Ernst, with thallium, a heavy metal that was once used to kill rodents until it was banned in the U.S. in the 70s.

O'Neill says she only heard Ernst talk about Hanmer once, after the two men got into an argument when Ernst moved into a sober house

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