At about 11 a.m., Fremont police received the envelope by mail, and the employee who received it immediately notified her supervisor, Officer Geneva Bosques said.
The envelope had no return address, Bosques said. Similar incidents were reported last week by police departments in Hayward, Union City, Berkeley, San Leandro, Oakland and San Francisco.
Bosques said Fremont police were already on alert and have been keeping an eye out for such a delivery since Friday.
The envelope was taken outside and caused no major disruption to the department, Bosques said.
The FBI has taken possession of the mail to analyze it, Bosques said.
Tests up to this point have been negative for anything hazardous, but sending a hoax letter is a serious federal crime, an FBI spokeswoman said.
The FBI is continuing to work closely with all the departments that received a suspicious letter. Any tips can be called in to the FBI tip line at (415) 553-7400.