The emails arrived overnight Monday into Tuesday.
They threatened the safety of hundreds of thousands of students in the nation's two largest school districts, promising that a violent plan already had been set in motion and raising the specter of guns and bombs inside numerous classrooms.
New York City officials opted to open their public schools on time Tuesday, calling the message an amateurish hoax imitating a popular television series.
But across the country in Los Angeles, Superintendent Ramon Cortines took a different tack, closing every school in his sprawling district in a move that disrupted the daily lives of more than 640,000 students and their families.
By Tuesday afternoon, the emails in Los Angeles were being investigated as a hoax designed to disrupt school, said Rep. Adam B. Schiff, the ranking Democrat on the House Intelligence Committee.
