Gargiulo, 43, was found guilty in August of two first-degree murder cases, of a willful, deliberate, and intentional attempted murder and attempted escape.
The jury, consisting of six men and six women, considered "several hours" before they reached the death sentence recommendation, the office said in a release.
A second murder victim, Mario Bruno, a neighbor of Gargiulo, was killed and maimed in December 2005.
In 2008, Gargiulo attacked and stabbed Michelle Murphy, who survived. He had accidentally injured himself in the attack and allowed investigators to associate his DNA with the other murders, KABC reported.
Gargiulo's sentencing is scheduled for February 28, the DA office said. The judge will make the final decision.
He was also charged with murder in the Chicago area in 1993, the DA office said. The prosecution claims that the victim, Tricia Pacaccio, was Gargiulos first. At the time, he was 17.
CNN has turned to Gargiulo's lawyer, but has not received a response.