A spate of apparent copycat threats in the aftermath of the deadly shooting rampage in a Minnesota school in March is wreaking havoc in scores of American schools, forcing many to cancel lessons, shut down classrooms and beef up security.
Incidents have been reported as far afield as California, New York, Texas, Arizona, Michigan, Illinois, Maryland, Virginia and Iowa.
The disturbing trend has been most disruptive in Minnesota itself, where in the past fortnight at least six schools have been closed after receiving ominous warnings.
"It's almost every day that a school has to close because of threats we can't tell are real or not," said Charlie Kyte, director of the Minnesota Association of School Administrators.
Ronald Stephens, director of the National School Safety Association, said the phenomenon mirrored an effect witnessed after previous high-profile school shootings. "After Columbine, there was a huge increase in threats," he said.
Nine people, including five students and a teacher, died at Minnesota's Red Lake high school before disgruntled student Jeff Weise turned his gun on himself. It was the second-worst school shooting in United States history, following the 1999 massacre at Columbine high in Colorado, which left 15 dead, including the two student killers.
Officials in Waconia, Minnesota, closed four schools last week following death threats, which included "hit lists" of students, teachers and police officers daubed on school property and contained in anonymous messages.
"It's not a decision made lightly," said schools chief Jerry Kjergaard.
"But if I'm going to make a mistake, it's going to be on the side of caution."
But malicious hoaxers may be exploiting jittery schools, said Mr Stephens.
Some threats have coincided with the dates set for school tests.
Earlier this month, staff at Mount Olive high school in Illinois discovered a chilling message written on the wall of a toilet block: "On May 9 I will bring a gun into school and kill at least 26 people."
The graffiti was signed "JJ", which is text-message shorthand for "Just joking."
Classrooms were locked, metal detectors installed, and the wall panel was removed and taken to a forensic science laboratory.
"It was quite unnerving," said schools chief George Wilkerson. Police subsequently arrested a 17-year-old student, who admitted it was a prank.