The case against Officer Derek Carlile went to trial earlier this month, but ended with a hung jury.
In court documents filed Tuesday, Snohomish County Deputy Prosecuting Attorney Lisa Paul asked that the charge against Carlile be dismissed because prosecutors do not "believe that another jury will be able to reach a unanimous verdict."
Carlile had been charged with second-degree manslaughter after his daughter, 7-year-old Jenna Carlile, was killed in the family's van on March 10.
Investigators said Carlile was off duty and headed to a wedding with his wife and children when they stopped at an art studio in Stanwood. Carlile and his wife left the children in the family van while they dropped off business cards. Carlile's loaded personal handgun was also in the van.
His 3-year-old son grabbed the weapon and unintentionally shot and killed Jenna, officials said.
Prosecutors argued that as a trained police officer, Carlile should have known the risks of allowing kids to be unsupervised around the gun, especially because his young son seemed fascinated with firearms. They said the child died because of Carlile's carelessness.
"No reasonable person would leave a loaded unsecured gun in a van with four children," Paul said during the trial.
Carlile's attorney called the shooting a tragic mistake, not a crime.
"Derek accepts responsibility and is at fault for leaving a firearm in his car," defense attorney David Allen said as the case went to the jury. "There's no question about it; Derek screwed up. He knows that."
Allen said the jury in the case was split, but leaning heavily in Carlile's favor.
"It came down to the question of criminal negligence. The majority of the jury felt that there was not criminal negligence, and it was not a crime; it was a mistake, one of those momentary lapses," he said.