Inside the courtroom, prosecutors laid out the charges against officer Derek Carlile.
"He did something that no reasonable person would do," said prosecutor Lisa Paul. "He did something that no person in their right mind would do."
Carlile is accused of putting a loaded revolver in a cubbyhole in the family van, then getting out and leaving his four young children inside.
Investigators say his 3-year-old boy grabbed the gun and shot his older sister while she sat in the back seat. The girl later died.
"This was an accident," said defense attorney David Allen. "This was a very unfortunate situation that had dire consequences."
Prosecutors say Carlile is a trained police officer and should have known the risks of allowing kids to be unsupervised around the gun, especially because his 3-year-old seemed fascinated with firearms. They say the child died because of Carlile's carelessness.
"The evidence will show that the defendant's guilt is complete," Paul said.
Carlile and his attorney don't dispute the facts in the case, but say it's wrong to hold him criminally responsible.
During Thursday's proceedings, jurors heard a recorded interview Carlile did with detectives. As the defendant listened, he burst into tears.
While prosecutors try to stick to the facts of the case, the defense wants jurors to think of the emotional toll of a father tragically losing his child and consider whether a lapse in judgement truly amounts to a crime.
"An open mind is crucial. Even more crucial now than perhaps we emphasized before," Allen said.
The prosecution rested its case Thursday afternoon and the defense will pick up on Friday. It's unclear if Carlile will testify.