OAKLAND — A jury found an Oakland man guilty Monday of murdering a 15-year-old girl at an Alameda park on Halloween three years ago.

Quochy "Tony" Tran, 19, could face life in prison when a judge sentences him Aug. 20 for fatally shooting Ichinkhorloo "Iko" Bayarsaikhan in what police say was a botched robbery.

Tran, who was 16 at the time of the slaying, was tried as an adult.

He broke down when he was found guilty of first degree murder at the Rene Davidson Courthouse in Oakland.

"We are extremely disappointed in the decision," defense attorney Anne Beles said. "We are also extremely disappointed in the Alameda County District Attorney's Office. He was 16 years old. He should never have been prosecuted from the very beginning."

Five other boys were convicted in juvenile court for their roles in the slaying at Washington Park on Central Avenue on Oct. 31, 2007.

On the night of the slaying Tran and his companions initially met up at Lincoln Recreation Center in Oakland's Chinatown and took a bus into Alameda, where they went trick-or-treating, police said.

The group became angry after someone traveling in a passing van "egged" them and they attempted at least one other robbery before they confronted Bayarsaikhan and her friends in Washington Park, according to investigators.

During the robbery attempt, one boy pulled the rifle from his coat and fired about five shots over the heads of the victims, causing the girl and her friends to scatter, police said.

As Tran and the others were then leaving the park, someone shouted the gun was not real, prompting some victims to begin following the suspects, according to testimony.

Tran then grabbed the gun and fired a single shot, hitting Bayarsaikhan in the back.

"This was a crime that tore the heart out of a community," prosecutor Tim Wellman said Monday. "The defendant is now finally being held accountable for his violent conduct, which took Iko's life."

Beles said she plans to appeal.

A native of Mongolia, Bayarsaikhan came with her brother and parents to Alameda six years ago. She was a junior at Alameda High School at the time of her death.

"My heart was shaking," said Uranchimeg Khishigdorji, the victim's mother, about the moments before the verdict was read. "If Tony was found not guilty, for the rest of my life I would be suffering."

Tran's family declined to comment.