Classes for about 650,000 students were canceled Tuesday after several people on the district's school board received an email threat. More than 2,700 officers were involved in walk-throughs at more than 1,500 school sites in searches for explosives or weapons, Los Angeles school district police Chief Steven Zipperman.
"We can now announce, and conclude, and tell you, and tell the community that we believe that our schools are safe, and we can reopen schools in the Los Angeles Unified School District tomorrow morning," Los Angeles Unified School District School Board President Steve Zimmer told reporters.
Authorities said they don't know who made the threat, but that the FBI determined it was not credible, Los Angles Mayor Eric Garcetti said. A member of the House Intelligence Committee said it appeared to be "a hoax or something designed to disrupt school districts in large cities."
A similar threat was delivered in New York, but those schools didn't close.
At the press conference, officials defended Superintendent Ramon Cortines' decision to close down schools for the day. Cortines did not speak at the press conference.
"This was not my decision to make but my decision to support," the mayor said.
"I think that anything can be viewed with a mirror into the past, but when decisions were made, the information that we have now was not available," Los Angeles Police Chief Charlie Beck said.
Cortines, speaking earlier in the day, said the "message" was received early Tuesday morning and 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).
Beck 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."
Some already at schools when closures announced
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'
Garcetti noted how the San Bernardino attack had changed the dynamic for dealing with terror threats 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.
The mayor said he hopes police catch the person who made the threat.
"Whether it's criminal mischief, whether it's somebody testing vulnerabilities of multiple cities, we still do not know enough to say definitively," he said. "What we do know is that it will be safe for our children to return to school tomorrow."
Beck was asked what would happen with the next threat.
"This is always an issue we deal with," he said. "We have suffered far too many school shootings in America to avoid this kind of threat. ... We take these very seriously, as we should. The public expects us to."