PALO ALTO — In an episode that has become increasingly frequent at Bay Area schools, a panicked Palo Alto High School went into lockdown for several hours Thursday in response to a phone threat that was later determined to be not credible, authorities said.

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Police would later determine that the caller used a stolen cell phone to make two calls to 911 around 12:31 p.m. and said he was going to “shoot up” Palo Alto High School “in 15 minutes,” according to a Thursday night news release. The suspect also gave the name of a Paly student who had no involvement in the scheme, in an apparent attempt to frame the student, police said.

A phalanx of Palo Alto police officers, joined by the Mountain View and Menlo Park police and the Stanford Department of Public Safety, “immediately flooded the school” and the surrounding area, authorities said. The school was locked down and students and faculty barricaded themselves inside their buildings and classrooms.

About 90 minutes later, the threat was deemed to be unfounded.

“Officers have determined that the phoned-in threat was likely a hoax,” Palo Alto police said in a tweet just after 2 p.m. “There was no violence and there were no physical injuries.”

Officers remained on the campus “out of an abundance of caution” and to offer a sense of safety and security to shaken students and staff, many of whom thought they were in the sights of a possible campus gunman.

Several hours after the scare, police said they traced the call to a cell phone that had been lost or stolen at the Town & Country Village shopping center. While the owner of cell phone had it deactivated, the device was still able to make emergency calls, which is ironically a common safety feature built into virtually all commercial cell phones.

Investigators also contacted the student whose name was given by the unidentified caller, and the student, who was among those in a locked-down classroom, was soon determined to have had “no involvement in this incident whatsoever,” police said.

Detectives are following up on several tips and leads provided by students and staff, police said, adding that any suspect tied to the threats will likely face both criminal charges and civil liability for the cost of the massive police response.

“(The threats) created a great deal of stress and anxiety for students, parents, school staff and the community in general,” police said in a statement. “The law enforcement response to this incident was significant, and took officers away from other important duties and calls.”

The Thursday threat is the latest in a series of disparate but similar threats in the region since the notorious Feb. 14 mass shooting in Parkland, Florida, that claimed 17 lives and spurred a national gun-control advocacy movement, numerous school walkouts and the March for Our Lives demonstrations.

Cupertino High School evacuated its campus Tuesday after a threatening phone call, and last week, police stepped up their presence around San Mateo High School after a man showed up at a Big 5 sporting goods store looking to buy a gun and said he wanted to “shoot up a school.”

In an unrelated incident also reported Thursday, San Mateo police responded to an office park in the 1400 block of Fashion Island Boulevard after a note containing an apparent bomb threat was found in a restroom. Officers and a bomb-sniffing dog from the San Mateo County Sheriff’s Office searched the building and found no evidence of any explosive, according to a San Mateo police news release.

Anyone with information about the Palo Alto High School threat can call Palo Alto police at 650-329-2413, or leave an anonymous tip by email at paloalto@tipnow.org or by sending a text message or voice mail to 650-383-8984. Tips can also be submitted on the police department’s mobile app that can be downloaded at www.bit.ly/PAPD-AppStore or www.bit.ly/PAPD-GooglePlay.