MINNEAPOLIS — The Internet messages were so vicious that they pushed 16-year-old Ivy Griffiths into therapy for depression.

“Go cut yourself until you bleed to death,” a Champlin Park High School classmate wrote to Ivy online. “You deserve to die,” wrote another.

Now, a new attack is mounting, but this time the targets are the cyberbullies themselves. The oft-scrutinized Anoka-Hennepin School District in Minnesota is launching a yearlong anti-cyberbullying campaign to address everything from sexting to the dangers of online gaming.

But in combating cyberbullying, which experts say has become the most prevalent form of bullying, law enforcement and school officials face an ever-changing challenge: The cruel messages and pictures Ivy received arrived on websites like Ask.fm, Snapchat, Instagram and Kik — relatively new sites that have been linked nationally to teen suicides.

“We know that low self-esteem is linked to cyberbullying, and most teens have heard that some of these high-profile suicides have been linked” to new social websites, said Justin Patchin, a University of Wisconsin-Eau Claire professor of criminal justice who has co-authored four books about cyberbullying.

“Teens still visit Facebook, but it is pulling back a bit,” said Patchin, a native of Hoyt Lakes, Minn. “There’s always a new site, like Formspring. And now, there’s software that deletes messages and images within seconds, and there’s lots of software online for resurrecting those disappearing images.”

Teenagers are discovering all of it — sometimes to torment classmates with comments and photos that are sent anonymously and vanish suddenly, before they can be traced.

At Anoka-Hennepin, Minnesota’s largest school district, three of seven students the district lost to suicide from 2009 to 2011 were identified by friends and family as having been bullied. An estimated 42 percent of all kids have been bullied online, according to information gathered by Anoka County Attorney Tony Palumbo. Patchin says about 20 percent of students are sexting — sending nude photos of themselves or classmates online — although many of the two dozen students interviewed last week at Champlin Park High School said more than half the student body participates in sexting, with some students sending “selfies” and others putting naked photos of their significant others on the Web.

“The term I’m hearing recently is sextortion — sexting and extortion, the threat to release those pictures,” said Victoria Powell, a Sherburne County, Minn., prosecutor who has crusaded against cyberbullying for years.

Powell started the county’s Bullying Prevention Project a decade ago. Today, the kind of bullying she often encounters stems from altering online photographs and texts, hacking, posing as someone else, repeating instant messages, pornography sites, tricking students into sharing passwords and creating negative websites.