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LA Unified officials said tonight that all district schools have been declared safe and will reopen tomorrow.

The decision was made after law enforcement officials determined that an email foretelling violent acts across the district was judged to be “not a credible threat” by investigators, in the words of Mayor Eric Garretti, who joined city and regional officials at an early evening news conference.

The announcement brought an end to one of the most challenging days in LA Unified history, causing anxiety and inconvenience to hundreds of thousands of families who send their children to the district’s 1,100 schools. The officials said 2,780 law enforcement personnel had swept 1,531 school sites to determine that all schools were safe for the resumption of instruction.

However, as Garcetti warned, the opening of schools does not bring an end to the episode. He objected to characterizing the email as a “prank” or a hoax,” suggesting instead it could be a case of “criminal mischief or testing true vulnerability of the district.”

“We sure hope we catch who is responsible,” he said. “At best someone was engaged in extreme criminal mischief, a serious crime. Somebody needs to pay for that. If they were testing our vulnerability, we did a pretty good job of responding.”

The mayor said the threat came to LA Unified’s board president, Steve Zimmer, at 10 pm Monday night. Zimmer immediately contacted law enforcement, and it quickly led to a collaboration involving the FBI, country agencies, the Los Angeles Police Department and the district’s police department.

After hours of work, they presented information early in the morning to Ramon Cortines, who had stepped down three days before as district superintendent, to make a decision about opening schools.

Cortines closed them, a decision that was roundly supported by the various law enforcement agencies, particularly in light of the terrorist attack two weeks ago in San Bernardino. Others questioned the decision as an overreaction, giving into fear.

“Anything can be viewed through a mirror to question the information we have now,” said LA Police Chief Charlie Beck. “It wasn’t available this morning.”

He added, “If you knew what superintendent knew would you have sent your child to school? Every parent I know I’ve talked to said no. People want to know you treat my kids like you do your own.”

Cortines said closing the schools was a decision made “out of an abundance of caution and to ensure safety and security in our schools. We now must get back to the business of educating our kids.”

He said schools would have crisis counselors available “for students and employees who may need additional support.”

He thanked all the agencies who works on the issue today, and the district identified some of them:  he FBI, the Bureau of Alcohol, Tobacco, Firearms and Explosives, the LA Police Department; LA County Sheriff’s Department; California Highway Patrol; the Departments of Police Services at the University of California and Cal State-Northridge; and the police departments from Long Beach, San Fernando, Bell, Gardena, Hawthorne, Huntington Park, Inglewood and South Gate.

Tom Torlakson, the state Superintendent of Public Instruction, said he was “nearly 100 percent certain” that the district could recoup about $29 million it would lose as a result of the loss of a day of instruction.

“I praise the exemplary coordination, you all should be proud,” Torlakson said. “I don’t know how you’re all still standing.”

Privately, after the press conference, Zimmer, who hadn’t slept all last night and was in meetings all day, said, “You can tell from the bags under my eyes and they way I’m talking that I’ve not had much sleep. I am surprised I’m still standing.”