Part of the Apple Park campus in Cupertino was evacuated after first responders discovered an envelope containing a white powder substance, NBC Bay Area reports. The Santa Clara County Fire Department’s incident report shows there was a fire alarm and hazmat response at Apple’s headquarters just after noon local time.

Emergency personnel since have determined that the situation is under control, and employees apparently have been allowed to go back inside, according to the publication. It’s unclear what the substance may have been.

Apple told employees at Apple Park that “authorities concluded that there was no presence of hazardous materials,” according to an email obtained later in the day by The Verge. The email says that operations at the campus are back to normal, and that “all sections are open.”

Apple did not immediately reply to a request for comment about what might be going on. After rescheduling return-to-office plans for its employees several times, Apple will start a “phased approach” to its hybrid work pilot in April.

Update March 15th, 7:19PM ET: Added incident report info from SCCFD.

Update March 15th, 8:42PM ET: Added info from Apple email.