First responders discovered an envelope containing a white powder material at the Apple Park facility in Cupertino, according to NBC Bay Area. A fire alarm and hazmat reaction were activated at Apple’s headquarters just after noon local time, according to the Santa Clara County Fire Department’s incident report.

Employees have presumably been permitted to return inside after emergency officials assessed that the issue is under control, according to the magazine. The substance’s identity is unknown.

According to an email acquired later in the day by The Verge, Apple assured staff at Apple Park that “authorities found that no dangerous chemicals were present.” According to the email, “all sections are open” and campus operations are back to normal.

An inquiry about what’s going on received no immediate response from Apple. Apple will begin a “phased approach” to its hybrid work program in April, after deferring its employees’ return-to-office plans multiple times.