When the message appeared on Ryan Dominick's smartphone, the 14-year-old paused to muster some courage. In it was a link sent by an unknown user that could contain anything from a flirtatious come-on to an embarrassing put-down.
It turned out to be a picture of Ryan photoshopped to make him look overweight, complete with multiple chins and engorged cheeks. Luckily, the athletic and confident freshman found the picture hilarious.
"LOL," he responded to the anonymous sender while actually laughing out loud and showing the picture to friends.
The picture was typical of the pranks exchanged among Ryan's Los Angeles classmates on the anonymous-messaging app Backchat, one of a fast-expanding breed of social-media apps that mask users' identities and can create messages that self-destruct. Anonymous and ephemeral, apps such as Whisper, Secret, Ask.fm and Snapchat fill a growing demand among teens for more fun, less accountability and more privacy online.
But the boom is opening secret new corners of the Internet at a time when educators and law enforcement officials are worried about the safety of youth online. As teens look increasingly for alternatives to the social giants Facebook and Twitter, the anonymous apps create the opportunity for bullying and cruelty in a forum where they cannot be tracked.
Educators, parents and law enforcement officials complain that it's hard enough to keep up with activity on public forums such as Facebook. Accounts on the anonymous sites are even harder to monitor, they say, noting that the popular anonymous question-and-answer forum Ask.fm has become a magnet for cyberbullying.
The apps fill a critical need, however, among teens, the majority of whom have their own smartphones and manage their social lives on multiple online networks. Many have been thoroughly lectured about the dangers of sharing too much on traditional sites: They know that future employers and college recruiters are likely to sift carefully through their Twitter and Facebook accounts.
Besides, when parents, grandparents and Little League coaches became core users of Facebook, kids naturally gravitated to new places where they could socialize away from the watchful eye of adults, experts say.
"Youth need a way to share material in a more natural way, like a voice conversation, and that they don't have to worry about lingering around and being part of what's now become curated life online," said Amanda Lenhart, a senior researcher at the Pew Internet and American Life Project.
Experts estimate that dozens of anonymous and so-called "ephemeral" apps such as Snapchat have sprung up, attracting millions of teenage users. Most are relatively simple, capable only of sending photos and texts, and there are no fussy profiles or privacy settings.
Take Backchat: The app was created by Ryan's 14-year-old classmate Daniel Singer and attracted 125,000 members in its first few months. He launched it with support from his father and $200,000 in seed investments.
"There's suspense in not knowing who is sending you messages, and it's actually kind of fun, knowing someone spent the time to make the photo," said Ryan, who has been on a hunt since last week to find the mystery sender of the picture. "It adds spice, more than on Facebook, where everything is permanent and out there."
Some apps are backed by the biggest Silicon Valley venture capitalists. Whisper, whose users post pictures and comments anonymously, received $21 million from Sequoia Capital last fall. Secret was created by former employees of Google and Foursquare, to take advantage of a backlash to the scripted feel of Facebook.
"What we found was people on social networks were trying to put forth their best image of their great dinners, amazing beach vacations. But life isn't always like that," said Chrys Bader, a co-founder of Secret.
The Secret app combs through users' contact lists to find other members of the anonymous network. A user never knows which of his or her friends might be a Secret user, too.
Many of the Secret posts are corny jokes, but some target individuals. One post invited condemnation of a girl identified by name: "Raise your hand if any of you have ever felt personally victimized by" the girl, it said.
Among the comments, came this reply: "Push her in front of a bus."
Mr. Bader said the site allows users to flag abusive comments and that harmful posts will be taken down. He declined to say how many people use Secret and whether they are mainly teens.
According to a 2011 Pew Internet and American Life study, nearly 9 of 10 teen users said they have witnessed "mean or cruel" actions aimed at peers online. Still, many experts say serious online cruelty is rare and that the risks of cyberbullying have been overblown because of a few high-profile teen suicides.
But they also wonder if the recent proliferation of anonymous apps could change that.
This month, Olivia Birdsong, a 13-year-old Memphis, Tenn., resident, saw classmates trash a girl as a "slut" on the question-and-answer board Ask.fm. The high school freshman said a few people stood up for the girl, but many piled on with criticism.
"The worst stuff happens on the anonymous sites because people are either too scared to say something to someone's face or they want to present someone with public humiliation," Olivia said.
Numerous psychological studies show conflict is often resolved when people talk face to face. When people can see signs of sadness or other emotions, they tend to back down. Facebook said the majority of users who are flagged for abusive or bullying conduct never do it again. On the anonymous sites, there are no such brakes on negative behavior.
To protect against cyberbullying, some of the new apps don't allow minors. Whisper, for example, requires users to be 17 or older. But no one polices the requirement. Meanwhile, it takes just one tap on a pop-up notice to verify age and get online.