Students and staff at San Bruno's Portola Elementary School are back to class after an investigation into a "suspicious circumstance" found no threats to the campus, according to police.
At about 1 p.m., San Mateo County emergency officials issued an alert and a shelter-in-place order FOR the school at 300 Amador Ave.
About 30 minutes later, San Bruno police announced on Twitter that a hazardous materials team conducted testing and determined "the substance" to be harmless.
No injuries were reported and authorities have not elaborated on what the substance was or what led to the alert.
Police and fire officials did not immediately respond to requests for information.