Apple’s Silicon Valley headquarters was partially evacuated on Tuesday after an envelope containing an unidentified white powder was found inside the facility, according to reports. 

The suspicious substance was found by first responders, who then ordered employees to evacuate, Santa Clara County Fire Department Capt. Justin Stockman told NBC Bay Area. 

It’s unclear what the white substance could have been, but white powders in envelopes bear a resemblance to anthrax attacks.

In an email to employees at its Cupertino, Calif. campus late Tuesday, Apple said that “authorities concluded that there was no presence of hazardous materials” and that operation were back to normal, according to The Verge. 

Apple did not immediately respond to a request for comment.

Under federal law, people who perpetrate hoax terror attacks can get prison time — even if the white substance turns out to be baby powder or another innocuous substance.

There was a fire alarm and hazmat response just after 12 p.m. at Apple’s office, according to a fire department and incident report obtained by the site. 

It’s unlikely many Apple employees were around when the envelope was discovered, since CEO Tim Cook has pushed back the company’s return-to-office deadline several times. Employees are now set to return in a “hybrid approach” starting in April.