FBI agents were working with Manhattan Beach police Friday to find the person responsible for making an emailed “specific threat” that forced school district officials to shut down Mira Costa High School as a precaution for the second time in a week.

The threat apparently was unrelated to offensive graffiti that showed up at Redondo Union High School, where officials kept the campus open Friday. Many parents, however, took their children out of school.

In Manhattan Beach, neither school officials nor police identified the nature of Friday’s threat, which also prompted the closure of Pennekamp Elementary School because of its proximity to the high school. On Monday, a telephone caller using a voice-altering device phoned in a bomb that closed the campus.

“An extremely beneficial result would be if the person(s) responsible for these threats were apprehended and prosecuted to the fullest extent of the law,” Manhattan Beach Unified School District Superintendent Mike Matthews wrote in a letter to parents Friday afternoon. “We are pleased to report that FBI agents are in Manhattan Beach working with MBPD on tracing both of these threats.”

Two parents called the district with tips that “might be valuable” for police, Matthews said.

Students reported on Twitter that their principal, Ben Dale, told them over a public address system that school was to be closed after they assembled for 7 a.m. “zero period.”

In an email to parents a short time later, Matthews said a threat “specific in nature” had arrived in an email and Dale “took quick action to close the school for the day.”

Matthews said no direct threats were made to any other schools, but Pennekamp was closed because of its location.

In his afternoon letter, Matthews described the threat as “very specific.” But, he said, police had “declared that no suspicious activity exists at Mira Costa High School” and school would reopen Monday. Manhattan Beach police will be present at Mira Costa and all district schools when students and faculty arrive, he said.

On Monday, police found nothing after officers from about 10 agencies responded to search the campus for hours.

The shutdowns have raised questions about whether closing school is the appropriate tactic in the face of such threats. Last year, district officials shut down the Mira Costa campus for two days following a threat on the “Yik Yak” anonymous posting app.

“I share the frustration that many of you have expressed at the disruption this has caused our school and our community,” Matthews wrote. “We lost two days of instruction at the high school last year and now two days this year. Our students, parents, and employees have had to deal with uncertainty and no small amount of stress.”

Matthews said officials reacted to the threats with great caution, and believe they made the proper decisions.

“This is the unfortunate nature of our world today,” he said. “We have to walk the fine line of not letting threats impact our way of life, while at the same time acting prudently to keep students and staff out of harm’s way.”

Carolyn Seaton, executive director of educational services at Manhattan Unified, said Mira Costa students were not taking final exams this week.

Meanwhile, Redondo Union High School officials kept the campus open for classes Friday after graffiti was found spray-painted in red on a sidewalk near the school cafeteria. The message read, “You are all (expletive) animals being led to the slaughter as you deserve,” police said.

Redondo Beach police Sgt. Shawn Freeman said no specific threats were emailed or phoned to the school. Early reports that the campus had been placed on lockdown were untrue, Freeman said. However, police increased their presence at the campus.

District officials notified parents about the graffiti, but classes were not canceled, Freeman said. Many parents, however, chose to pull their children out of school for the day.

“Essentially, there’s not really a threat to the kids. Most of us are considering it a prank, but better safe than sorry,” said parent Susi Kaplan, as she picked up her daughter. “It’s really frightening when your daughter sends you a picture of the graffiti that was on the school grounds. It’s probably a prank and I’m trying not to panic about it.”

Parent Tammie New said her daughter sent her photographs of the graffiti at Redondo Union, and she acted immediately.

“My daughter sent me pics, the school locked down and I went to pick her up,” New said.

Matthews said how schools deal with threats “merits a larger conversation between our city, parents, students, and all other stakeholders.” He said he and his leadership team will attempt to respond to every email they receive from parents.

Experts say school administrators should assess threats on a case-by-case basis when determining whether to shut down campuses.

Ken Trump, president of the National School Safety and Security Services, a Cleveland-based consulting firm, said often administrators react to threats before assessing them, when it should be the other way around.

“They’re responding emotionally rather than cognitively,” he said. “They’re well-intended but the best practices are contrary to emotional, knee jerk reactions.”

Automatically evacuating schools after receiving vague threats, Trump said, is not necessarily a safer call.

“When you evacuate a school, you lose control of your ability to supervise kids and keep them in more secure locations,” he said. “If there’s not a clear, credible threat based on an assessment, you may actually be putting kids in less safe situations.”

Ronald Stephens, director of the Westlake Village-based National School Safety Center, said the more specific a threat is, the more serious.

“Generally, if they mention the type of bomb, the place, when it’s supposed to go off, once you start getting into details like that, it becomes much more credible,” he said.

Perpetrators are very often current or former students or others connected to the schools, Trump said, but sometimes they are made by people far away “swatting,” a trend in which hackers make false reports and trick emergency operators by having a victim’s address appear, sometimes resulting in a SWAT team arriving.

Trump said voice-altering devices are often used in swatting.

In Manhattan Beach, police identified a young female suspect in last year’s Yik Yak threats, but the department never announced charges.

Late this week, however, Sgt. Paul Ford confirmed the girl “did have to face the courts,” though he would not share any other details because she is a minor. She was not a Manhattan Unified student.