NORTH HOLLYWOOD, LOS ANGELES -- Police officers and crisis counselors were on hand Wednesday as Los Angeles Unified School District students returned to school following an anonymous email threat of a large-scale jihadi attack that prompted a district-wide closure.
LAUSD Police Chief Steve Zipperman said all officers at LAUSD will be in uniform the remainder of the week for high visibility.
The threat, which came in the form of an email to a school board member, impacted 1,500 campuses and 640,000 students.
MORE: Full text of threatening email sent to LAUSD
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.
LAUSD Superintendent Ramon Cortines said the San Bernardino attack that left 14 people dead influenced his decision to cancel classes.
The threat "was not to one school, two schools or three schools," he said at a news conference Tuesday morning. "I, as the superintendent, am not going to take the chance with the life of a student."
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."
Beck says the email was routed through Germany but that police believe its origin was much closer.
MORE: LAUSD schools deemed safe after threat investigation
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.
Maria Ruiz, an administrative assistant at Fair Avenue Elementary School in North Hollywood, called the situation stressful.
"I was here at about 6:20. Immediately, we called our operations office and confirmed that the threat was real, and I started sending out my phone messages for the school to all the parents and staff at 6:45," she said.
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.
"As you know, L.A. Unified always puts student safety first. I want to reassure students, parents, guardians, teachers and other employees that our schools are safe," LAUSD Chief Deputy Superintendent Michelle King said.
ABC News and the Associated Press contributed to this report.
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