Police say the 18-year-old tried to pin the shootings on his younger brother, but the evidence told a different story.
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What he says next may be the difference between life and death.
Robert Cotter hasn't said a lot in court.
"Are you entering a general time waiver at this time?" Judge Gary Hoff asked him Tuesday.
"Yes," Cotter said.
"Do you understand what a general time waiver is?" the judge asked.
"Yes," Cotter said.
That was the extent of his courtroom comments.
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Police arrested the 18-year-old in July, two months after the deaths of Patrick and Brian Cotter - his father and his 15-year-old brother.
RELATED: Teen accused of killing father, brother faces possible death penalty with family support
Robert initially told police it was a murder-suicide committed by Brian.
His remaining family told Action News they just want a straight story about what happened.
"(We're) trying to figure out why this happened," said an aunt who wanted to remain anonymous. "What we could've done to stop it. And I love Robbie and I hope he knows he's loved. But we just don't know why."
Legal analyst Tony Capozzi says Cotter's defense attorney will probably get a psychiatric review to help unravel the mystery.
"Was there animosity between the father and the son?" Capozzi asked. "Was there any domestic violence? There are a lot of factors that could come into play."
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Cotter's mental health, his mother's recent death, and his relationship with his father will all be factors.
Capozzi says prosecutors may use the death penalty in negotiations over a possible plea or punishment, but given Cotter's age, they might need more to get a death sentence.
"If more facts come out: was he lying in wait? Was he waiting at home with a gun, ready to kill his dad and kill his brother? Those are two circumstances that lead to a death penalty," Capozzi said.
Cotter is jailed with bail set at $3 million.
He's due back in court in November.
His family told Action News they don't doubt he did it, but they want him to have a full defense.