NEW YORK (NYTIMES) - A 14-year-old freshman at Murrow High School in Brooklyn was sitting in history class one April morning when she got a string of chilling texts from a friend.

A threat to shoot up the school had been posted on the chat site Omegle - and it included a list of about a dozen students who would be killed.

One of them was the 14-year-old girl.

"To see your child's name on a literal hit list was truly the most completely devastating thing," said Ms Jessica Heyman, the girl's mother.

But the girl, whose name is being withheld, knew immediately that the threat was a hoax: Just days earlier, another threat had targeted students at another New York City high school, the Clinton School, using precisely the same language.

The incidents at Murrow and Clinton were two in a string of nearly identical hoax threats aimed at more than a dozen New York City schools over the last four months, and at least nine other schools nationwide, including ones in Long Beach, California, and Hicksville, New York, on Long Island, according to parents, students and two senior law enforcement officials.

The New York schools include many of the city's most elite public and private schools, including the Berkeley Carroll School, the Brooklyn Friends School and Brooklyn Technical High School in Brooklyn, and Beacon High School, LaGuardia High School and the United Nations International School in Manhattan.

As recently as this week, police said, a threat was made against New Utrecht High School in Brooklyn.

Mr John Miller, the deputy commissioner for the Police Department's intelligence division, said the department is investigating seven of these threats in New York City, and coordinating with the FBI, which is probing the threats nationally.

"These are not credible threats," Mr Miller said. "They're meant to cause disruption."

Authorities believe the threats are made by a person - possibly overseas, Mr Miller said - who finds the names of students at a school by searching Instagram for children with public accounts using rudimentary social media skills.

Often, they pose as a student of the school which they are threatening, Mr Miller added.

The threat-maker targets high-profile schools to gain attention but does not appear to have any intention of following through, according to a separate senior law enforcement official, who spoke anonymously because he was not authorised to discuss the threats.

"We take every security related incident seriously to ensure the continued safety of our students and staff, and we are working closely with the NYPD on their investigation of these threats," said Ms Jenna Lyle, a spokeswoman for the Department of Education.

For decades, American schools have had to contend with fake fire alarms, bomb threats and threats to commit school shootings.

But these hoaxes reflect a disconcerting new reality for a country already reeling from a mass violence epidemic: Social media has made it increasingly easy to craft eerily specific threats of violence that clog up one of the few avenues law enforcement has to police them.

"If the system becomes overwhelmed by false alarms, some could slip through," said Prof Ron Avi Astor, a professor of social welfare who studies school violence at the University of California Los Angeles.

"It takes away a big tool."

The site where the hoax threats were made, Omegle, was also used sometimes by the gunman who killed 21 people at an elementary school in Uvalde, Texas.

The hoax threats posted on Omegle about New York City schools mentioned the type of assault rifle that would be used in a shooting and the music that would play: Abba.

The prevalence of hoax school shooting threats - and an uptick after a particularly notorious or deadly mass shooting - is not uncommon.

For most of this school year, the city fielded an average of about two school shooting threats a day, the senior law enforcement official said.

In the week following the Uvalde shooting, the number spiked to about six per day.

"Only a small percentage of these threats are serious. Others will make threats as a prank or in an effort to be disruptive, not unlike previous generations that would pull a fire alarm or make a prank phone call," said Prof Dewey G. Cornell, a professor of psychology at the University of Virginia who studies youth and violence.

"The stakes are higher now with social media and the tremendous anxiety that is generated by the threat of a school shooting."

After the hoax directed at Berkeley Carroll, a private school in Park Slope, Brooklyn, circulated in early February, the school increased security and allowed students to attend remotely for several days. But it did not close or lock down the school, telling parents it was following recommendations from the police.