MARTINEZ -- An "all for one, one for all" mentality led a group of young gang members to kill a 16-year-old Oakley boy at an Antioch house party for high school students, a prosecutor told jurors Monday.

Leon Moreno, Ricky Mendoza and Tony Martin, all 20, are on trial on murder, attempted murder and gang charges in connection with the Aug. 20, 2011, shootings in which Martin Navarro was killed and a 17-year-old female party guest was wounded in the shoulder.

In opening statements Monday, Deputy District Attorney Danielle Douglas said Navarro was killed because the defendants believed him to be a rival gang member, but it was unclear why the teenage girl was shot.

The shootings happened at a birthday party for twin boys, whom Douglas said are not affiliated with any gang. Their family members were hanging out inside the house, while 30 to 40 teenagers were in the garage with a DJ playing loud music.

The prosecutor said the gang jumped Navarro, and Mendoza shot him inside the garage after his ex-girlfriend, Antioch resident Jessica Juarez, pointed him out as a "snitch." Juarez, along with alleged gang members Christopher Donaldson and Carlos Guzman, is awaiting trial in the case.

As Navarro lay dying, dozens of teens ran screaming from the party. Martin then fired at a car full of teenagers, wounding the girl, Douglas said.

Defense attorneys said much of the prosecution's case is built on the testimony of George Hellums, a former gang member now in a witness protection program. They said his self-serving account of what occurred is in direct conflict with the statements by other witnesses.

Defense attorneys said there is no evidence that their clients were armed and forewarned jurors of conflicting evidence as to who was armed and inside the garage that night.

Contact Malaika Fraley at 925-234-1684. Follow her at Twitter.com/malaikafraley.