PHOENIX — Nearly two years ago, a 16-year-old boy who had accompanied two siblings to a west Phoenix park was fatally shot as he defended his younger sister from a man who police say had targeted them for an armed robbery.

Authorities say Andrew Murphy was killed after he stepped in front of the robber to confront him after he had approached Murphy's 14-year-old sister with a demand for money. Police say the gunman ran away, threw his revolver under a vehicle and was arrested about a quarter mile away.

Jury selection is scheduled to begin Monday for 20-year-old Vincent Quimayousie on charges that he killed Murphy and made earlier armed robbery attempts at Marivue Park on Sept. 18, 2012.

Quimayousie has pleaded not guilty to first-degree murder, armed robbery and aggravated assault charges. In a police interview, he acknowledged being near the park on the day of the killing, but denied involvement in the crime, according to court records. If convicted of the murder charge, Quimayousie would face a life prison sentence.

"We had to accept the fact that he is gone," said Dominic Murphy, father of the victim. "We had to dig deep into who we are as a family."

Amy Kalman, one of Quimayousie's attorneys, didn't return a call seeking comment Friday.

Quimayousie is accused of trying to carry out other armed robbery attempts of people at the park just before he approached the Murphy siblings.

Andrew Murphy, who was at the park with his 14- and 11-year-old sisters, was shot twice and was pronounced dead at a hospital about an hour later.

Police said Quimayousie fled on foot and was seen banging on the door of a nearby house. When approached by officers, he threw a bandanna on the ground, pulled a gun out of his waistband, ducked behind a vehicle and was seen manipulating the firearm. He was arrested after a struggle with officers, police said.

Prosecutors said several witnesses couldn't positively identify Quimayousie because he was wearing a bandanna over his face during the crimes. Still, at least two other people who were at the park were able to identify Quimayousie.

Quimayousie's attorneys said in court records that their client's fingerprints weren't found on the weapon and that a defense expert will testify that there's no scientific proof that the gun found under the vehicle was the same firearm used to kill Murphy.