A shelter in place was ordered at San Miguel Elementary School in Sunnyvale Friday morning.
Authorities said a threat was phoned into the school before 9:30 a.m. Friday and the school was locked down has a protective measure. There were no reports of injuries.
The City of Sunnyvale tweeted that a SWAT team responded to a "threat" on the campus but specific details of the incident were not being released.
Authorities gave school officials the all clear around 1:30 p.m.
The incident was sparked when the principal of the school received three calls Friday morning with unspecified threats against the school.
The Sunnyvale Police Department was notified and they were on campus by 9 a.m., going room to room searching for a threat.
Just before 1 p.m. authorities were sending in a robot to search underneath the portable classrooms on campus.
Parents were notified early on in the morning and told they could pick their children up at Fair Oaks Park. Those that were not picked up early were scheduled to be released shortly after 2 p.m.
By 10:45 a.m., police officials said they had searched about half the school and found no threat. The neighboring schools were alerted to a potential threat but not locked down.