LOS ANGELES (Reuters) - Public schools in Los Angeles reopened on Wednesday, a day after local officials canceled classes for all 640,000 students in the nation’s second-largest school district over a threatened attack with bombs and guns that was later deemed a hoax.

Returning high school students found themselves thrown back into the throes of final exams as the semester ended, with police providing a stepped up presence at campuses to reassure those left jittery by the unprecedented shutdown.

Authorities conducted an extensive search on Tuesday of the Los Angeles Unified School District’s more than 1,000 schools, and by late in the day said the buildings were safe and students could return. The investigation to find the sender of the emailed threat continued on Wednesday.

The email, which authorities said was routed through Germany but likely originated locally, came nearly two weeks after a married couple inspired by Islamic State fatally shot 14 people and wounded 22 others at a county office building 60 miles (100 km) away in San Bernardino.

A similar email was sent to New York City’s public schools, though officials dismissed it as a hoax and kept campuses open.

The regular morning bustle was in full display at Venice High School on Wednesday as school buses dropped off students and other kids spilled out of parents’ cars.

“I feel my daughter is totally safe at school today,” said Trish Halfacre, dropping off her 14-year-old. Halfacre said she backed the officials who ordered the closure.

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“Better safe than sorry,” she said.

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Several students outside the Ramon C. Cortines School of Visual and Performing Arts, a downtown campus named after the superintendent who made the decision to close all campuses, said they also supported the one-day interruption.

“For all we know, it could have been our school or any other ... that could have been bombed and a lot of kids would have gotten hurt,” said 10th grader Kaniah Chapman.

Officials also said they made crisis counselors available for students and distributed materials to teachers intended to help them lead class discussions on the disruption.

New York City Police Commissioner William Bratton and federal officials who asked not to be identified described the decision in Los Angeles as an overreaction.

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But Los Angeles Police Chief Charlie Beck on Tuesday defended the school district’s decision to keep students and staff home out of an abundance of caution.

“It is very easy in hindsight to criticize the decision based on results that the decider could never have known,” Beck said.

The threat, emailed late on Monday night, came from someone who claimed to be a devout Muslim prepared to launch an attack at multiple schools using bombs, nerve gas and rifles, Brad Sherman, a Democratic U.S. congressman from California, told the New York Times.

In response to a separate hoax made against a middle school in Farmington, New Mexico, police on Tuesday arrested a 13-year-old boy suspected of threatening a bombing, authorities said. The school was placed on lockdown but not closed.

The United States has experienced a series of attacks at schools in recent years. The deadliest in the past decade occurred in 2007 at Virginia Tech, where a student gunman killed 32 people.

A 20-year-old gunman in 2012 killed 20 young children and six educators at Sandy Hook Elementary School in Newtown, Connecticut.