MANHATTAN BEACH ( — Threats prompted three schools in the South Bay to be closed on Friday.

Classes at Mira Costa High School at 701 S. Peck Avenue in Manhattan Beach were cancelled after someone called in a bomb threat in the morning, police said.

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Nearby Pennekamp Elementary School at 110 S. Rowell Avenue was also closed as a precaution, according to the Manhattan Beach Unified School District.

After a search, police determined there was “no suspicious activity” at either campus.

Classes will resume as usual (with a) “continued police presence at all Manhattan Beach schools on Monday as a precautionary measure,” police said.

On the same day in Redondo Beach, someone scrawled graffiti on the grounds of Redondo Union High School, said Redondo Beach police Sgt. Shawn Freeman. It read: ‘You all are (expletive) animals being led to the slaughter you deserve.’

The graffiti was spray-painted in red on a sidewalk near the school cafeteria, but no specific threats were emailed or called in to Redondo Union High, Freeman said.

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Anxious parents rushed to campus to take their children home. “What happened in San Bernardino, it’s not worth taking a chance. We’re going to just scoop them up and get them safe,” said one parent.

Police believe the threats at Mira Costa High and Redondo Union High were not related.

But they are part of a troubling trend. There has been a surge in threats to schools, hospitals and colleges in the wake of the San Bernardino massacre.

Four days ago on Dec. 7, a “non-specific” threat prompted authorities to evacuate Mira Costa High School and cancel classes for the day, Manhattan Beach policed said. A search of the campus turned up nothing dangerous.

On Dec. 7, Carter High School in Rialto was locked down after a bomb threat. Police later determined it was a hoax “caused by two students playing a joke on each other,” Rialto police said.

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On Dec. 8,  authorities responded to two bomb threats in the Inland Empire – one at Grand Terrace High School, the other at Loma Linda University Medical Center. Those threats also turned out to be hoaxes.