Pedro Espinoza is accused of capital murder for allegedly carrying out the March 2008 shooting of 17-year-old Jamiel Shaw in the commission of a gang activity.
Espinoza is a known gang member and undocumented immigrant. The case has sparked controversy over how police handle immigrant suspects.
The defense's position was that although Espinoza bears the tattoos of the 18th Street gang and BK, signifying Blood Killer, the prosecution has scant evidence to prove Espinoza is guilty of murder.
"If they had a gun and it was found in my client's back pocket, trouble. But they don't have a gun, they don't have a gun that's linked to him," said Espinoza's attorney, Csaba Palfi.
On the stand was LAPD Sgt. Brian Churchill, who described a scene that brought tears to Shaw's family members and friends. He said the victim's father, Jamiel Shaw Sr., ran out of the house to aid his son and was hysterical to see him shot.
As for what prompted the deadly shooting, prosecutors allege it was Shaw's Spiderman backpack, which was red and was interpreted as a gang color.
Chrystal Miles testified she was on the phone with Shaw when she heard a stranger's voice, a chilling challenge asking the boy, "Where are you from?" Miles said that she then heard a gust of wind and the phone disconnect.
Viewing opening statements from the front row were Shaw's parents, including Sgt. Anita Shaw, who served in the Army and was in Iraq when the shooting happened.
"It was very difficult. It's like a knife turning in your stomach, it is very painful knowing that my son was murdered for no reason. It hurts," she said.
Since the murder, his father has led a campaign to deport gang members who are in the U.S. illegally.
Shaw was a junior at Los Angeles High School and had been its football team's MVP three years in a row.
"To see all that training and hard work, staying out of trouble, to end with a funeral? It's just unacceptable," Shaw Sr. said.
The trial is expected to last up to four weeks. If convicted, Espinoza could face the death penalty.