FORT WORTH, Texas — Teenagers text-messaging explicit photos of themselves or classmates is undeniably a mammoth lapse in judgment.

But is it a crime?

It's a question that experts say has confounded law enforcement agencies nationwide as the trend known as "sexting" grows in popularity. It's also one that authorities in Tarrant County, Texas, will have to answer after allegations were made earlier this month at a Keller district middle school.

A girl alerted her mother about a group of students at Hillwood Middle School who were sending one another explicit photos of female students, according to Fort Worth police. Some parents said the students involved were in a club called The Cause.

The mother notified officials at the school. School officials then contacted police.

Investigators have obtained photographs from students' cell phones and will turn them over to Tarrant County juvenile prosecutors, who will determine whether they are evidence of a crime, said Sgt. Pedro Criado, a police spokesman.

In a letter sent to parents, the principal explained that sexting can be a felony.

Sexting has already led to prosecution of several teenagers across the U.S. for obscenity, even child pornography. One Florida teen was placed on a sex offender registry.

The severity of the charges has led some states to create specific misdemeanor charges related to sexting among teens. This summer, the American Civil Liberties Union sent letters to prosecutors, lawmakers and educators in Ohio urging them not to pursue criminal charges against teens for sexting, calling the punishment too harsh.

Under Texas laws prohibiting child pornography, someone in possession of a nude picture of someone under 18 can go to prison or juvenile detention. Texas Attorney General Greg Abbott warned in May that teenagers' transmissions of explicit electronic images could meet the definition of child pornography and that teens could face 10 years in prison.

About 20 percent of teen girls have sent nude or seminude photos electronically, according to a study by the National Campaign to Prevent Teen and Unplanned Pregnancy.

"One consequence that no one fully expected to come out of this growing trend is the legal entanglements," said Bill Albert, spokesman for the group, which released a report on sexting this year. "I think what you are seeing is attorney generals and law enforcement agencies trying to figure out what to do about this stuff."

The investigation at Hillwood was not the first in North Texas involving inappropriate text messages.

In March, Fort Worth police investigated four students at Northwest High School after nude photographs of a 16-year-old female classmate were thrown in the air in a crowded hallway.

The photos were first sent to a student by cell phone, police said. The message was forwarded to at least two other students, one of whom printed the photos and made about 20 copies.

Police at the time said the students could face obscenity charges — a Class A misdemeanor that could be enhanced to a state jail felony because there were more than a half-dozen copies. Authorities, however, declined to pursue charges.

In an unrelated case in Denton County, a Sanger Middle School student spent a night in juvenile detention last year for having a nude photograph of a 13-year-old girl on his cell phone.

Tim Bednarz, chief of the Tarrant County district attorney's juvenile division, said his office has reviewed several sexting cases but has determined that none rose to the level of a crime. Overall, he said, the cases remain rare.

Prosecutors consider the circumstances of each case before deciding what charges could apply, he said. The possibilities range from Class A misdemeanor obscenity charges to felony child pornography charges.

For child pornography charges to apply, however, a provision in the Texas Penal Code requires that the defendants be at least two years older than the victims, he said.

If they are not, prosecutors could still pursue other charges, he added.

"It would also depend on what exactly the images were depicting," Bednarz said. "There are provisions in the penal code that deal with basic public decency and displaying material that is harmful to a minor. It just depends on the facts of each case."

Teens would be wise not to take any chances, he said.

"Those engaging in that activity are treading in dangerous water," he said.

The possibility of arrest for sexting catches most teens by surprise, Albert said.

"The emotional and social consequences are pretty clear," he said. "But they tend not to see possible criminal charges coming."

Some prosecutions have drawn national headlines. An 18-year-old Orlando, Fla., man who sent a naked photo of his 16-year-old girlfriend to her friends and family after they argued was convicted of a felony count of sending child pornography. He was sentenced to five years' probation and was required to register as a sex offender.

In Pennsylvania, three teenage girls successfully sued a prosecutor to prevent him from pursuing felony charges after racy photographs were found on their classmates' cell phones.

The impact on victims has also been documented. The hanging suicide of an 18-year-old Ohio woman was blamed on nude photos of her that her ex-boyfriend allegedly forwarded to her classmates after they broke up.

The rash of cases has led lawmakers in several states to propose laws meant to guard against excessive punishment while still deterring teenagers from sexting. In Vermont, for example, minors charged with sexting are now dealt with in juvenile court instead of facing sexual-exploitation charges and being required to register as sex offenders.

Legislation was also passed in Utah and Ohio setting misdemeanor charges for juveniles who distribute pornographic material.

"We don't like it if two consenting 16-year-olds are exchanging naughty pictures; it's not a good thing," Albert said. "But the question is whether it is something that should follow them around for the rest of their lives. That seems like an awfully heavy sword."

Smart, promising students are getting caught up in the trend. Kevin Clancy, a Dallas attorney who handles juvenile cases, said he has not yet defended any clients charged in sexting-related cases but has been contacted by worried parents.

"You would be surprised: Eighth-grade, straight-A, beautiful girls are sending these photos to boys," he said. "And, of course, the first thing those boys do is send them on to other people. Parents are worried."

Abbott, the attorney general, has touted education as a powerful tool against sexting and has urged parents to have frank conversations with their children about the consequences, both legal and social, said Jerry Strickland, his spokesman.

"This is an emerging issue that has impacted schools across the state," Strickland said. "Parents should pay close attention to their kids' use of technology."