Garcia said the crime was Doody's idea and that the two wanted to steal gold and cash that they believed the monks kept. Authorities said the robbers ransacked the temple's living quarters and made away with about $2,600 and other valuables.
Each victim was shot in the back of the head.
Doody faced 20 counts, including robbery and burglary and nine charges of first-degree murder. Prosecutors have already indicated they plan to put Doody on trial again.
During the retrial, Doody's attorney urged jurors to discount Garcia's testimony, saying he was a "sophisticated and savvy" teenager at the time who lied to minimize his involvement.
Prosecutors told the panel that the evidence showed both Doody and Garcia were responsible for the killings.
Doody's brother and mother were members of the temple. He has maintained his innocence.
The 9th U.S. Circuit Court of Appeals overturned Doody's conviction and ruled his confession inadmissible partly because he wasn't properly read his rights.
In the confession, Doody said he went to the temple during the robbery but claimed he was outside when the shootings occurred.
The appeals court's decision meant prosecutors couldn't use Doody's confession at his retrial. They instead relied largely on Garcia's testimony.
Garcia said Doody was determined to leave no witnesses and shot each victim. Items taken from the temple were found at Garcia's house, where Doody was staying at the time.
The judge at Doody's first trial spared him the death penalty, noting it couldn't be determined beyond a doubt whether Doody was the triggerman.
Prosecutors couldn't seek the death penalty in Doody's retrial because of a 2005 U.S. Supreme Court decision that prohibits authorities from pursuing the ultimate punishment against defendants who were under 18 years old when the crime occurred.