(CNN) -- Classes were canceled Tuesday for the Los Angeles Unified School District because of what the superintendent called a "rare" threat that came amid new concerns about security nationwide.
But there were no initial reports of explosives or weapons being found in the schools, and the second-guessing began.
A similar threat was delivered in New York, but the schools didn't close. Meanwhile, a member of the House Intelligence Committee said the threat appeared to be "a hoax or something designed to disrupt school districts in large cities."
An "electronic threat" received early Tuesday prompted the decision to close facilities to nearly 700,000 students, Los Angeles school district police Chief Steven Zipperman said, adding that the threat "is still being analyzed."
Superintendent Ramon Cortines said the "message" referred to backpacks and "other packages." He said many schools were threatened, though none by name. The threat was toward students in schools (as opposed to on buses).
The threat came in emails to several people on the district's school board, after which police became "very concerned," Los Angeles Police Chief Charlie Beck said. He told reporters that the email -- which was routed through Germany, though it's believed to have originated closer to the United States -- mentioned the use of explosive devices, assault rifles and pistols.
Cortines said his school district often receives threats. While he didn't go into detail, he said recent events -- such as this month's massacre in nearby San Bernardino, California; the Paris terror attacks; and heightened concerns about potential terrorism across the United States -- factored into the cancellation.
"The circumstances in the neighboring San Bernardino, I think what has happened in the nation, I think what happened internationally" played into the decision, Cortines said. "I, as superintendent, am not going to take the chance with the life of students."
The superintendent said he's asked authorities to search all of the roughly 900 charter and K-12 schools in his district "before the day is over." He promised a statement that could offer more information about what prompted his decision and lay out what will happen next, including whether classes will be in session Wednesday.
That won't happen, Cortines insisted, "until I know it's safe."
Some already at schools when closures announced
On any given day, running the Los Angeles Unified School District is a complex operation. It has more than 650,000 students enrolled in kindergarten through 12th grade, with some 250,000 more in adult education programs.
Educators are tasked not only with teaching lessons, but nurturing a safe, positive school environment. Then there's the matter of getting students to and from school as efficiently as possible.
These challenges grew even more complicated when Cortines announced the school closures at about 7:15 a.m. PT (10:15 a.m. ET). By then, some facilities had opened, and some students and staff were already there, with others on the way. They've all since been asked to go home or, in the case of children, stay under an adult's watch until someone can come get them.
To help, the Los Angeles public transit system offered students with valid IDs a free ride home through noon.
Cortines pointed out that district schools often go into lockdown in response to nearby real or potential dangers. The threat in this case was "not to one school, two schools, three schools. It was many schools, not specifically identified."
"That's the reason I took the action that I did," he said.
New York police think similar threat a 'hoax'
New York Police Commissioner Bill Bratton said a superintendent in his city's school system -- the country's largest with 1.1 million students over 1,800 schools -- received an email "almost exactly the same" as one sent to Los Angeles.
New York police think it is a "hoax," not a credible threat, according to Bratton. The message is believed to have originated overseas but does not seem tied to a jihadist initiative, the commissioner said, noting Allah -- the Arabic word for God -- is not spelled with a capital A.
The FBI is working with New York as well as Los Angeles police on the threat, according to Bratton.
Calling the writing generic, New York Mayor Bill de Blasio said it's important that people don't overreact.
"(It would be a) huge disservice to our nation," the mayor said, "to close down our school system."
U.S. Rep. Adam Schiff, a Democrat from California and a member of the House Intelligence Committee, issued a statement: "While we continue to gather information about the threat made against the Los Angeles and New York School Departments, the preliminary assessment is that it was a hoax or something designed to disrupt school districts in large cities."
Schiff said the safety of children is paramount, but "in an environment in which it is very easy to transmit threats, real and otherwise, and when fear and disruption may be the goal as well as the effect, communities and law enforcement will need to make a difficult judgment as to how to respond in a variety of circumstances."
L.A. mayor grateful for 'abundance of caution'
In Los Angeles, city leaders rallied behind Cortines' decision to call off school because of the threat.
Mayor Eric Garcetti noted how the San Bernardino attack had changed the dynamic in Southern California. "An abundance of caution is something that ... all of us who have children can appreciate," said Garcetti, who noted the threat suggested that weapons were already in place at a number of school campuses.
"We will continue to hope that this is nothing and that our children can be back at school tomorrow," the mayor said. "But as a parent and as a mayor, certainly, I'm here to support this school district."
Beck echoed that sentiment, saying, "Southern California has been through a lot in the recent weeks. Should we risk putting our children through the same?"
Cortines told reporters he took action after consulting with other top school officials and the head of the school district's police. After that, Los Angeles police got involved and "connected some of the dots nationally on this," according to Garcetti.
"There are no secrets," Cortines said. "Somebody has sent information that leads us to pause and make sure that ... our children and our staff is safe."