MINNEAPOLIS (WCCO) — Three days after a gruesome double murder in Minneapolis, homicide charges were filed against the victim’s son, who is also the uncle of the other victim.

Ishmael Roberts, 22, was returned to Minnesota after he was arrested in Waterloo, Iowa. That’s where investigators say he fled after stealing his dead mother’s car.

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Authorities know what he used in the killing — but still don’t understand what prompted the attack.

This was by all accounts an absolutely gruesome murder scene. Investigators believe Ishmael Roberts used a samurai sword to kill his mother, Beatrice Wilson, and then turned the sword on his 14-year-old nephew, Peter Wilson.

It had been two years since Roberts had visited his mother and family in their north Minneapolis home. But when he arrived around 3 a.m. on Monday, the visit turned terribly violent.

“Cause of death is clearly multiple strokes of a sharp instrument,” said Mike Freeman, Hennepin County Attorney, “we believe was a sword from the sheath left behind.”

After killing his mother, investigators say Roberts turned the sword on his nephew. Both were dead in upstairs bedrooms when police arrived.

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Blood led to the garage where Beatrice Wilson’s car was missing. When he rolled it in Waterloo, Iowa, no sword was found.

“Probably a very good chance he threw it out on the road somewhere between here and Waterloo Iowa, whether we find that, I don’t know,” said Capt. Amelia Huffman, of the Minneapolis Police.

The complaint against Roberts says he replaced his blood-covered shoes at a Waterloo Target store.

Store cameras clearly show him tossing the bloody shoes in a dumpster, along with the car’s blood-covered floor mat. Yet, what continues to baffle investigators, is what could have prompted such a vicious and deadly act?

“We asked him one question — what was this about with your family? He said as is his constitutional right, ‘I don’t want to talk,'” Freeman said. “We may never know.”

County Attorney Freeman says there is only slight evidence that Robert may have struggled with mental illness, but there’s nothing concrete that could ever explain his rage towards his own mother and young nephew.

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The investigation will continue.