A teenager accused of murdering his girlfriend's mother and stepfather came to the couple's Compton home with a "murder kit" that included a baseball bat, a mask and rubbing alcohol, a prosecutor told jurors Tuesday.

Giovanni Gallardo executed a "sneak attack," strangling his girlfriend's mother and ambushing her stepfather with the bat before stabbing him with a knife, Los Angeles County Deputy Dist. Atty. Eric Siddall said during opening statements of Gallardo's murder trial.

The prosecutor said Gallardo and his girlfriend drove around in her mother's Jeep looking for a place to dump the bodies. In Long Beach, they dug a grave for the girls' stepfather, Jose Lara, but the ground was too hard to make the grave large enough to include Gloria Villalta's body, Siddall said.

For the next two days, the young couple kept her body in the back of the Jeep as they shopped for supplies for a Halloween party, Siddall said. Villalta was eventually buried at a vacant lot in Norwalk.

"It’s almost unbelievable that someone could be so callous to two human beings, to murder them in their own home and to treat their bodies so poorly," Siddall told jurors.

Gallardo, who was 16 at the time of the killings but is charged as an adult, faces life in prison if convicted of the October 2011 slayings.

The bodies were found buried in shallow graves. Lara, 51, was handcuffed and his body covered with a blanket. Villalta, 58, was bloated from decomposition and her head was wrapped in duct tape.

The prosecutor told jurors that Gallardo initially told sheriff's detectives that Villalta had disappeared and that Lara had threatened him with a gun. But Gallardo, now 18, eventually told detectives in detail about the killings and took authorities to where the victims were buried and their vehicles abandoned, Siddall said.

Gallardo's attorney, Deputy Alternate Public Defender Scott T. Johnson, told jurors during a brief opening statement that they would learn "about a terrible, tragic series of events" but that he was confident they would find his client not guilty after listening to the evidence.

"Keep an open mind," he said.

Before the jurors were brought into court, Johnson confirmed to Superior Court Judge Ricardo R. Ocampo that he was not declaring a doubt about his client's mental competence to stand trial but said a defense expert considered Gallardo to be a "borderline" case.

Johnson asked the judge to give Gallardo additional time during the trial to confer with his lawyer if need be when witnesses were being questioned. The judge agreed.

Gallardo's girlfriend, Cynthia Alvarez, was convicted earlier this month of both murders. 

During her trial, Alvarez, 16, blamed her boyfriend for the crimes. She claimed she did not want her parents dead but did not seek help because she was afraid of her boyfriend, whom she described as abusive.

A jury deliberated for about three hours before convicting her of first-degree murder for both deaths.

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jack.leonard@latimes.com

Twitter: @jackfleonard