Police in Utah made a gruesome discovery on New Year’s Eve, finding a 41-year-old mother and her 9-year-old daughter dead inside their home in Magna following an apparent murder-suicide that may have occurred as long as three weeks ago.

Investigators believe that Karina Clark fatally shot her daughter, Madison Clark, before fatally shooting herself, PEOPLE confirms.

Officers from the Unified Police Department were summoned to the Clark residence on Sunday after someone noticed that a Christmas present placed on their porch went unclaimed for days, according to Unified Police Lt. Brian Lohrke.

Lohrke told reporters during a press conference Monday that investigators believe the mother and daughter died between one to three weeks before their bodies were discovered.

Police did not indicate a potential motive.

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“This is one of those situations where we’re kind of left in the dust right now, trying to figure out why a mother had the need to kill her 9-year-old daughter,” Lohrke said. “It’s a question we may have forever.”

PEOPLE was unable to reach any of Clark’s relatives for comment.

Lohrke said he believes Clark may have suffered from mental illness.

Gibson Clark, Madison’s oldest brother, spoke to Fox 13 in Salt Lake City, and said his mother was “really troubled.”

Gibson added: “We all grew up with her and we called child protective services on her quite a bit.”

Gibson also said that his mother’s behavior, which was sometimes violent, led him and his siblings to put some distance between themselves and her.

“I think there was stuff going on in the house that we didn’t know about,” said Gibson.

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Neighbor Ben Cummins, who said he lived next to the Clarks for about a year, told the Salt Lake Tribune, said he didn’t see anything alarming about Clark’s behavior, calling her a “warm person” with a “good heart.”

“I said, ‘How could somebody do that to their child, to anybody?’” Cummins told the outlet.

A YouCaring page, established by a family friend to fund funerals for the mother and daughter, states Clark also had twins named Bailey and Brandon Miller.

“The twins have already lost their father earlier in childhood to suicide and now again through this situation,” the page states. “Mental illness is such an ugly and heartbreaking thing!! You never know what someone is going through and capable of and my heart is breaking for these kids who now have to bury their mother and little sister.”