An attacker forces a 17-year-old to call her parents for cash -- but a "secret code" could have saved her life.

Seventeen-year-old Lily Burk was abducted in downtown Los Angeles this past weekend -- and her attacker forced her to call her parents twice to ask how to withdraw money from an ATM using a credit card. Her parents said she didn't mention she was in trouble -- and two hours later, she was dead.

It's a parent's worst nightmare -- how did they not know Lily was in distress? The answer is heartbreakingly deceiving.

"You have a kid who's calling you, and called twice -- that may have been reassuring," explains former L.A. County Deputy District Attorney and momlogic expert Robin Sax. "She's where she said she was, doing what she said she was doing."

Lily had told her parents she was at an ATM to withdraw money to buy shoes. Instead, she was allegedly at the hands of Charlie Samuel, a 50-year-old transient with a history of violence.

Police say Lily was approached at 3 PM on Friday at her car near Southwestern University School of Law, where she had gone to pick up some exams for her mother, an attorney who worked there.

Between 3:35 PM and 4 PM, Lily placed those two calls. Her parents told her to come home -- but she never did. They filed a missing persons report at 5:25 PM Friday evening; her body was found Saturday morning. Samuel was arrested for her murder on Sunday.

Could Lily have signaled to her parents that she was in trouble? "We don't know if he was dialing the phone, holding the phone, if he had a knife -- we don't know those things," explains Sax. "It's easy for us to judge from the outside."

One lesson to take from this tragedy, says Sax, is the importance of creating a secret language or code between parents and children. "Every family needs to have a code word of 'Call the police; I'm in danger,'" says Sax. "That was the communication opportunity."

In fact, ATM may be the perfect code word. As Sax explained, ATM heists are common, and no one should ever be talking or texting on the phone while using one. Children should make a deal with their parents that they'd never call from an ATM. If they do, it's likely they're in a heist -- and the red flag has been raised.

"This is an opportunity for parents to think about how we conduct ourselves," says Sax. "You've got to be completely aware of your surroundings." That means no cell phones and no texting -- "one of the things perpetrators like is people who are distracted" -- and parents should teach their children even to look confident. "Walk with your shoulders and head up ... if you look confident, they'll move on to the next potential victim." And one can never be too cautious. "We are so programmed to give people the benefit of the doubt ... but we compromise safety for political correctness," explains Sax.

But what can be done once the attacker has the upper hand? Sax says parents should instruct their kids to "scream for help and clock him in the eyeballs with your keys." Take it one step further by learning self-defense -- and at all costs, avoid being alone with an attacker in a car. "Once you're in a vehicle with someone, the control and power dynamic changes," says Sax.

Police say Lily, a student at Oakwood School in North Hollywood with dreams of becoming a writer, was beaten to death inside her car.

"The thing we want people to know about Lily is that she was a beautiful person, and that she was looking forward to her life. She was funny, warm, kind, and empathetic. She was deeply and widely loved," reads a statement from her parents.

Here is a timeline of Lily's murder, from the L.A. Times:

2:30 PM Friday: Burk leaves her home in the Los Feliz area of L.A. to pick up some exams at Southwestern University School of Law, close to downtown. She was collecting the exams for her mother, an entertainment lawyer who also taught at the school.

3 PM: Samuel allegedly approaches Burk at her car, which was parked near the law school. Samuel then allegedly abducts Burk.

3:35 PM: Samuel attempts to withdraw money from at least one location in downtown Los Angeles with Burk's credit card. But the girl's credit card was not set up to be used at an ATM.

3:35 - 4 PM: Burk calls both her mother and father, asking how to withdraw money using her credit card. She does not say she is in distress.

4:52 PM: Samuel allegedly exits the driver's side door of Burk's Volvo at 458 S. Alameda St. in downtown Los Angeles.

5:25 PM: LAPD officers see Samuel in downtown L.A. drinking beer and in possession of a crack pipe. He is arrested. Burk's parents report her missing. A missing persons report is filed and detectives begin searching for her.

6:15 AM Saturday: Workers find Burk's "lifeless body" inside her car. They call 911 and police arrive.

Sunday morning: Police say they matched fingerprints inside Burk's car to Samuel.

Sunday evening: Samuel is arrested in connection with Burk's slaying.

Robin Sax is a former Los Angeles County Deputy District Attorney who specialized in prosecuting sex crimes against children. She is the author of six books including "Predators and Child Molesters: What Every Parent Needs to Know To Keep Kids Safe." Robin is a regular legal commentator on Larry King Live, Nancy Grace, Fox News and has a weekly radio show, "Justice Interrupted." Robin lives with her husband and three children in Los Angeles, California.