Capital murder defendant Juan Cantu told a fellow gang member-turned-police-informant that he planned to only shoot his friend's ex-girlfriend when he went to the woman's Dallas trailer home in January 2004.

But Mr. Cantu was surprised when her son answered the door, and Mr. Cantu discovered his intended victim also had a new roommate, according to an audio recording played at Mr. Cantu's trial Thursday.

As a result, prosecutors say, he killed the boy and his mother, and then raped, stabbed and set the roommate on fire on the morning of Jan. 10, 2004. But she survived and identified him during the trial in a Dallas courtroom earlier this week.

On the recording, the informant asked Mr. Cantu how old the boy was.

"He saw my face. Would it matter?" Mr. Cantu said on the recording. "I have kids. I'm a father, and it would kill me. But he saw my face. Does it matter how old he was? But to answer your question, he was 15."

In the recording, Mr. Cantu also described how he tied up the boy, Adrian Martinez, in the living room and shot him in the head three times. His mother, Bonita Mendoza, was in a back bedroom sleeping, where she was shot.

"I tap [sic] her three times in the back of the ear," Mr. Cantu said.

Prosecutor Brandon Birmingham told jurors that the motive for the shooting was that Ms. Mendoza was leaving her boyfriend, Christopher Montalvo, a friend of Mr. Cantu's.

"He wasn't paying me to do it. But I was doing it because that's how I'm taking care of my family. If he got [expletive] up, I got [expletive] up," Mr. Cantu said on the recording.

Mr. Montalvo also faces a capital murder charge in the deaths of Ms. Mendoza and Adrian and remains in the Dallas County Jail.

Paul Brauchle, Mr. Cantu's attorney, called two witnesses Thursday who placed his client in San Antonio visiting a friend the same weekend as the attack.

Vania Torres told the jury that she and Mr. Cantu were together the entire weekend from Jan. 9 through Jan. 12, when he returned his rented SUV to Houston.

"We were never apart for more than five or 10 minutes," she said.

In addition to the surviving victim's identification of Mr. Cantu in court, Mr. Birmingham also presented evidence that DNA from a rape kit matched Mr. Cantu's.

On the informant's recording, which was taken from a device on his key chain, Mr. Cantu expressed shock that the woman had survived.

"I hit her at least eight times with the blade, with a [expletive] at least 6-inch blade," Mr. Cantu. "I felt her take her last breath. And it didn't come out of her mouth or nose. It came from her body. It just went shhhh, and she was out."

The jury was not told about a South Texas case with some connections to Mr. Cantu's trial.

A couple of weeks after Ms. Mendoza and her son were killed, Ms. Mendoza's 19-year-old daughter, Michelle Moreno, also was shot and stabbed near Houston.

Ms. Moreno and Michael Montalvo, a relative of Christopher Montalvo's, were found in a Cadillac just outside Rosenberg in Fort Bend County, said Detective Michael Kubricht of the Fort Bend County sheriff's homicide division.

Christopher Montalvo was indicted in that double shooting.

Closing arguments are scheduled to begin this morning in the 194th State District Court. Prosecutors are not seeking the death penalty. If convicted, Mr. Cantu would receive a life sentence.