Police are frantically searching for two young girls in Houston, Texas, who never made it to school Tuesday morning—and fear a troubling Facebook “game” might be behind their disappearance.

The girls, 13-year-old Mary Tran Le and 12-year-old Tianny Granja, were last seen around 7:30 a.m. walking to the bus stop for Owens Middle School, but they never made it on board.

While their families fear the pair may be in danger, they’re also concerned the six-graders are participating in a harrowing internet game: the 48-Hour Challenge.

“They were talking about the challenge the other day,” Dayanna Gomez, Tianny’s mother, said. “I just want to find my daughter, and I hope she is just hiding and not in danger.”

Martinez was first alerted to her daughter’s disappearance when a school receptionist called to say Tianny never made it to class.

“It is the school protocol to call the parents when children don’t show up for school in the morning,” a spokesperson for the school said, declining to comment on whether the game was popular among Texas teens. “That’s what we did on Tuesday morning.”

The parents immediately called the girls, but both of their cell phones— which are “always on them”— were turned off.

“That’s when I knew something was wrong,” the 12-year-old’s mother said.

Le’s parents could not be reached for comment.

The 48-hour Challenge, which originated on Facebook, is simple: It encourages teenagers to go “fake missing” for two days in the name of social-media notoriety, as they earn “points” with every social mention about their disappearance, authorities say.

South Carolina authorities believe they fell prey to the challenge in January during a frantic countywide search for 13-year-old Diana Clawson, who didn’t show up for school one Wednesday.

About 24 hours after she was reported missing, the teenager was found hiding under a bed surrounded by shoes.

Clawson’s mother, Tonya, has repeatedly denied her daughter was involved in the Facebook-driven campaign.

“Police take missing children cases very seriously, and we always immediately send out a ton of resources because we know that every second counts,” a spokesperson for the Rock Hill police told The Daily Beast Thursday. “This challenge is extremely dangerous because it takes away resources for another person in need.”

That incident prompted the Charlotte-Mecklenburg Police Department in North Carolina to issue a public warning about the challenge the next day, encouraging parents to talk to their children about the dangers of social media.

“Runaway and missing person reports are very serious matters and to use these attention seeking type challenges pulls law enforcement away from their many other duties and causes unnecessary anxiety amongst the families and friends that are involved, ” their warning on Facebook said.

While authorities do not know the challenge’s precise origins, a similar dare dubbed “Game of 72” popped up in Europe four years ago after a 13-year-old girl went missing.

According to local reports, the girl walked into her home three days later and told her parents and authorities she had just been “playing the game.”

“Parents need to be aware of a new ‘game’ traveling around social media involving teens daring each other to vanish for 72 hours,” a Michigan police department tweeted in response to that story in 2015.