America's two largest school districts received similar threatening emails on Tuesday, but reacted in opposite ways: New York shrugged it off, while Los Angeles shut down its system.
The conflicting responses reflected starkly different atmospheres in the two cities.
In Southern California, authorities remain on edge following a Dec. 2 massacre in San Bernardino. Los Angeles Unified School District Superintendent Ramon Cortines invoked that attack in explaining his abrupt decision to order 640,000 students to stay at home — a determination made over objections of some authorities there, one senior law enforcement official in Los Angeles told NBC News.
Cortines, whose term as superintendent will end in a few days, defended his unprecedented move. "Based on past circumstances, I could not take the chance," he said.
In New York, home to more than 1 million public school students, Mayor Bill de Blasio and Police Commissioner William Bratton held a news conference of their own, saying the anonymous message seemed bogus, and appeared to have been sent to other cities as well.
The threat "was so generic, so outlandish, and posed to numerous school systems simultaneously" that it was clearly not credible, de Blasio said. He added that it would have been a "disservice" to keep students home.
Bratton, a former chief in Los Angeles, suggested that officials there had overreacted. Seeming bemused, Bratton said the person behind the threat may have been a fan of the television show "Homeland."
Later, Los Angeles Mayor Eric Garcetti and Police Chief Charlie Beck stood by Cortines, saying it was wrong to chastise officials who acted quickly in the hours before the school day began.
"I think it's irresponsible based on facts that have yet to be determined to criticize that decision," Beck said.
The email arrived at the city's school board Tuesday morning, multiple sources familiar with the investigation told NBC News. According to a senior law enforcement official, the writer claimed to be a high school senior in Los Angeles who had been bullied and said he planned to attack schools with guns and bombs.
"It was not to one school, two schools, or three schools," Cortines said. "It was many schools, not specifically identified."
Rep. Brad Sherman of California, a member of the House Foreign Affairs Committee, said in a statement that he had seen the Los Angeles message, and that the writer also claimed to be a Muslim extremist who had teamed up with local jihadis. The writer did not appear to understand Islam very well, however, Sherman said.
The email that reached New York was routed through the same IP address, law enforcement sources told NBC News. There were many similarities, but New York authorities, consulting with the FBI, concluded that it was a likely hoax, because the student was simultaneously claiming to be a student in both cities, the senior law enforcement official told NBC News. The misunderstanding of Islam also raised a flag.
Many parents in Los Angeles — and many officials — said they understood what Cortines did, and expressed lingering fear from the attack on San Bernardino, where a radicalized husband and wife shot 14 people to death at a holiday office party.
The Los Angeles Unified School District operates over 1,000 schools and employs nearly 60,000 people. Garcetti said students would be permitted to ride city buses and trains for free.
The threat itself — focused on the school district, and no other institutions — is still being investigated and vetted, officials said.
The cancellation of classes was made early enough in the morning that many children had not yet left home or arrived at school. But Cortines said that the district was making sure that administrators would help students return home.
Even so, the impact of the last-minute closure rippled through the city, as thousands of working parents were forced to attend to their children or find backup care.
Meanwhile, Cortines said, he ordered district "plant managers" to examine school grounds for "anything out of line."
The district would decide late Tuesday afternoon if the schools are safe for students to return Wednesday, Cortines said.
Soon after the closure was announced, a 17-year-old student at Los Angeles International Charter High School was struck and killed by a city bus, authorities said.
This story originally appeared on NBCNews.com