"She was two months into being 13 when she died, only 2 months into being 13," Riling tells 48 Hours Mystery correspondent Troy Roberts.
After seven years, the pain still feels fresh. Riling's niece Chrissie was an honor student at her Catholic school in Danbury, Conn., and head of her cheerleading squad. But secretly, on her computer, she started meeting grown men in online chat rooms.
"One guy said that he was 'sorry that he hadn't worn a condom the last time they were together' and I was totally shocked," Riling says. "This is my little girl - they were talking about condoms. And at that moment I was so upset with myself for not checking the e-mails. I can't help but blame myself for not checking where she was going online."
In an interview with 60 Minutes II, her convicted killer, Saul dos Reis, a 24-year-old married man who worked in his family's restaurant, called Chrissie "a friend" whom he had met online.
"I was just trying to make another friend," .
Dos Reis strangled Chrissie after a sexual encounter, leaving her nude body in a stream.
Riling says when she hears of other crimes on the Internet, like the Craigslist killing, her mind goes to her niece "every time."
While extremely rare, there have been several murders involving people who answered ads on Craigslist.
Two years ago, Katherine Olson, 24, of Minneapolis showed up at a house after answering an ad on Craigslist for a job as a nanny. The 19-year-old man her later said he did it for no other reason than "he thought it would be funny."
In March 2009, 47-year-old New York radio reporter George Weber was allegedly killed by a 16-year-old he met on Craigslist after placing an ad looking for rough sex.
One North Carolina blogger, who calls himself "Trench Reynolds," has created a Web site devoted solely to tracking Craigslist crimes.
"In the past two years I've chronicled over 500 crimes as related to Craigslist, the majority of them would be prostitution, scams, harassment rapes, robberies and assaults," he says.
Helen Willis was planning a move from Texas to New York. "Initially, I was on Craigslist because I was trying to find apartment," she says.
The person placing the ad told her to send $3,000 as a down payment, which she did. Guess what happened to that money.
"I just feel so stupid, like I've been completely duped. It just makes me mad," she tells 48 Hours.
Considering Craigslist is viewed 22 billion times a month in 50 countries, the amount of crime is tiny. Craigslist CEO Jim Buckmaster says one reason is the users themselves police the site.
"If enough users flag that ad it gets taken down automatically, and obviously, we look at flagging activity to look for patterns to see if there's a misuse or abuse of the site," he says.
"As far as I'm concerned, that's like the inmates running the asylum," says Reynolds. "People who are going into Craigslist for illegal activity are not gonna flag ads, because that's what they are going into there for. And the people looking at Craigslist to, say, buy a couch aren't going into the erotic services section so they can flag illegal ads."
In spite of all the headlines, Internet safety expert Parry Aftab says we should keep it all in perspective.
"Craigslist is not to blame for murders of people who are meeting people on the site any more than the New York Times would be to blame or your local phone company phone book would be to blame if somebody reaches out, engages someone, shows up and kills them," she says. "This isn't an Internet issue. This is really the assault and battery and murder of someone on a crime spree."
When Aftab first heard about the Craigslist killing, she had two reactions: "One, that everyone would assume that if you meet someone or do anything with anyone one Craigslist that you're going to be murdered. And the other was that it's going to raise awareness."
"The Markoff case has captured the imagination of Connecticut and the country in a way that is very unique and offers a unique opportunity to pressure Craigslist to do the right thing," says Connecticut Attorney General Richard Blumenthal.
That's what Blumenthal and 40 other attorneys general are attempting to do. They've already persuaded Craigslist to charge $5 on a credit card for anyone who posts an ad for erotic services. That way, the posters are easily identified and traceable.
Craiglist says that it has already significantly reduced the number of questionable ads. But why not, as some suggest, remove erotic ads entirely?
"The erotic services category is there for legal operating businesses - escort services and massage parlors, etc., that are willing to operate on the right side of the law," says Buckmaster. He says whenever a crime has been committed, Craigslist works closely with law enforcement, as it did in the Markoff case, to track down the offender.
"When criminals try to use the site, they quickly find that Craigslist is an extremely hostile environment for them," he says.
The online world has changed a lot since 2002, when Shelly Riling's niece was so tragically killed. Or maybe it hasn't.
"When you're on the computer, it's like having another door in the house, Riling says."I mean, you lock your front door and back door before you go to bed, but you don't think about the computer; the computer is the door to the world. So the predators come in and they expose the vulnerability and they make the most of it."