LAUSD Superintendent Ramon Cortines said that the district received a threat in the form of an email that mentioned backpacks and other objects.
According to LAPD Chief Charlie Beck, the email was very specific to LAUSD campuses and it threatened violence. He said the email's "implied threat" involved explosive devices and the "specific threat" was a shooting attack using "assault rifles" and "machine pistols."
Later, district officials, along with L.A. Mayor Eric Garcetti, held a news conference where they announced all campuses were determined to be safe and will reopen Wednesday. The threat was said to be "non-credible."
A statement was issued by Cortines thanking the community and all officials that took part in the investigation.
The statement read in part: "I want to thank our L.A. Unified families for your support and your sacrifice as we worked through a very difficult day. I am also very grateful to my colleagues on the Board of Education and to our principals, plant managers and police officers who played a pivotal role in securing our schools."
Threat came from outside the United States
Two law enforcement officials familiar with the investigation told ABC News that the email threat came from outside the country and named "a fairly large number" of specific schools.
Beck says the email was routed through Germany but that police believe its origin was much closer.
Cortines called this threat "rare" and said the decision to close all schools for the day was made out of an abundance of caution. He said the Dec. 2 attack in San Bernardino that left 14 people dead influenced his decision.
"I, as the superintendent, am not going to take the chance with the life of a student," Cortines said at the news conference.
All LAUSD campuses were searched
Along with the closures, the district said all after-school activities had been canceled. In total, 1,531 sites were given walkthroughs by 2,780 law enforcement officers in the course of six or seven hours, according to Steve Zimmer of the Los Angeles Board of Education.
The closures led to a loss of $29 million in state funding, but State Superintendent of Public Instruction Tom Torlakson said funds could be reimbursed.
Beck said he agreed with Cortines' decision to close all schools following the threat. The chief said criticizing the decision to close LAUSD schools is "irresponsible."
"It is very easy in hindsight to criticize a decision based on results that the decider could never have known," Beck said at a news conference. "It is also very easy to criticize a decision when you have no responsibility for the outcome of that decision."
Several LAUSD school bus drivers told Eyewitness News that they were called back by their dispatchers to their respective bus depots Tuesday morning.
All students who were already at their schools were sent back home. Parents were urged to make arrangements to pick up their children if they were already dropped off on campus and also bring proper identification.
"We need the cooperation of the whole of Los Angeles today," said school board President Steve Zimmer. "We need families and neighbors to work together with our schools and with our employees to make sure our schools are safe throughout today."
Los Angeles Mayor Eric Garcetti had released a statement regarding the LAUSD closures, urging families to remain calm. He said the city's Emergency Operations Center has been activated and the MTA will provide free bus rides to all LAUSD students.
The FBI was aiding in the investigation led by Los Angeles School Police Department.
Other school districts around Southland not affected
Because the threat was specific to the LAUSD, schools in Pasadena, Burbank, Glendale, Altadena, Torrance, Simi Valley, Culver City, Glendora, Santa Monica-Malibu, El Segundo, Hermosa Beach, Newport Beach, Costa Mesa and Palos Verdes Peninsula were not affected.
All schools in Hawthorne, except for Cimarron Elementary School, which is part of LAUSD, are also open as scheduled. Los Angeles Trade Technical College, Los Angeles City College and Pierce College said their campuses would also be open.
Similar threat made against New York City schools
A similar threat against New York City schools was made via email Tuesday morning and it was very similar in nature to the email threat against LAUSD. Police in New York deemed it not a credible threat and did not close schools.
The LAUSD is the nation's second largest school district. The district includes 640,000 students in kindergarten through 12th grade and more than 900 schools and 187 public charter schools.
The district spans 720 square miles including Los Angeles and all or part of more than 30 smaller cities and some unincorporated areas.
If you have any questions regarding the closure, you can contact the LAUSD at (213) 241-2064 or at www.lausd.net.
ABC News and the Associated Press contributed to this report.