LOS ANGELES — Los Angeles public schools were abruptly shut down and students sent home Tuesday after police received what officials described as a credible bomb threat against the nation's second-largest school system, throwing into disarray the lives of millions of Angelenos — students, parents, teachers and other school staff members.
The threat was made electronically, and it was explicitly "to students at schools," said Ramon C. Cortines, superintendent of the Los Angeles Unified School District. He said, "some of the details talked about backpacks and other packages."
But Cortines declined to say more about the nature of the threat or how it was received. The Los Angeles Police Department and the FBI were investigating.
"It was not to one school, two schools or three schools — it was many schools, not specifically identified," Cortines said at a news conference shortly after 7 a.m. "I am not taking the chance of bringing children any place, into any part of the building, until I know it is safe."
Cortines and other officials said the schools would remain closed until the police and school administrators had searched every building to make sure the campuses were safe. But the logistical task involved is immense, as is the potential for chaos: The Los Angeles Unified School District has more than 640,000 students, enrolled in 900 schools and 187 public charter schools, sprawling across more than 720 square miles.
The district asked parents not to send their children to school, and ordered most employees to stay away, but many were already on their way, or even at school, when the order to close was made. Cortines and other officials said that the children were being kept out of the buildings and asked parents to pick them up at school gates. Much of the district's fleet of big yellow buses had already begun morning rounds, before being told to turn back.
New York City officials said that they had received a similar threat to schools Tuesday, but had concluded that it was a hoax. Mayor Bill de Blasio said Tuesday that he was "absolutely convinced" that there was no danger to schoolchildren in New York.
"Our schools are safe," the mayor said. "Kids should be in school today. We will be vigilant. But we are absolutely convinced our schools are safe."
Police Commissioner William J. Bratton, speaking at a news conference alongside the mayor, said that threats were often made against New York City schools and that they were assessed on a case-by-case basis. There was no indication, he said, that the threat made Tuesday was credible.
The Los Angeles school closings came as the region remained on edge after the terrorist attack in San Bernardino less than two weeks ago that left 14 people dead and 22 wounded. Over the past two weeks, there have been a number of bomb threats. But authorities treated this one differently.
"This is a rare threat," Cortines said. "We get threats all the time."
Although he would not elaborate on why this one prompted such a strong response, he cited circumstances like the San Bernardino massacre and other events around the world, an apparent reference to incidents like Islamic State attacks in Paris.
Bernys Maldonado, who lives in the Koreatown section of Los Angeles, said she was luckier than most, able to leave her three sons — in kindergarten, fourth grade and sixth grade — with her mother, and still head off to her job as a medical assistant. But with everyone's plans askew, she said, traffic was a nightmare.
"We were just preparing the lunches and about to walk out the door, when one of our teachers texted all the parents, and informed us all the schools were closed and to look at the news," she said. "Honestly, I thought the teacher was making a mistake or exaggerating. Never in my imagination did I think there would be something like this."
John Guanzon, 16, a junior at Hollywood High School, arrived unaware at the nearly abandoned school campus Tuesday expecting to take final exams. "When are finals going to be now?" he asked. When asked what he was going to do instead, he said, "I'm just going to stay home, I'm going to study."
Steve Zimmer, president of the school board, said: "We need the cooperation of the whole of Los Angeles today. We need employers to show the flexibility that a situation like this demands, and we ask you to show the maximum possible flexibility with your employees who are primarily mothers and fathers and guardians."
Cortines said administrators and police would "work systematically through the schools."
"I've asked the plant managers to walk the school, and if they see anything that is out of order to call the police," he said. "Not to touch anything, but if they see anything out of line to contact the proper authorities."
Steven K. Zipperman, chief of the district's police department, said the threat was specifically against the Los Angeles district. He said he knew of no threat made against any other schools in the region.
Nancy Vinicor, a fifth-grade teacher at Clover Avenue School in west Los Angeles, said she and her colleagues had been stunned first by the text alerts they received telling them to stay home, and then by the realization that it applied to the entire city. But their primary reaction, she said, was worry for their students.
"There are some kids who walk to school on their own," she said, "and we're concerned that some parents didn't hear the news and dropped their kids off. And the biggest concern is now the kids are going to be afraid to go to school. It makes me angry, honestly. We have students from all over the world, some of them come from places with horrible violence, and they thought they had escaped this kind of thing."
Michael Rosner, principal of the Gardner Street School, outside of Hollywood, stood in front of his elementary school Tuesday, the doors closed, and the street — normally packed with parents dropping off children — deserted. Rosner said the school district's early alert system had succeeded in telling parents not to come to school.
"Any parent that arrives here we are sending home," he said. "But our emergency system is working well."