Up to 50 people have been quarantined after an employee opened envelope with a suspicious powdery substance in an United States Customs office in Alameda, California.

A 2:43 pm emergency call was made reporting the employee feeling sick immediately after opening the envelope, KNTV reported.

Two more people also felt sick. Fire officials say their symptoms are not deadly, but the workers are receiving treatment under quarantine, and a HAZMAT crew is on location telling others to stay away from the area.

Fire officials established a “hot zone,” an area where safety equipment is required to avoid the risk of infection.

Police Lt. Jill Ottaviano told El Cerrito Patch that some people were eventually allowed to leave the building.

“Once we identify what it is,” Alameda Fire Department Spokesman Juan Medrano told the San Francisco Chronicle. “we can figure out how to deal with the substance itself and what levels of decontamination are needed.”