The building also houses Clear Channel Communication radio stations, including KFI-AM (640), KIIS-FM (102.7), KBIG-FM (104.3) and KYSR-FM (98.7).
The employee opened an envelope and found a powdery substance on the letter regarding Proposition 8, which voters passed Nov. 4, banning gay couples from marrying.
“It appeared to be against Proposition 8,” he said.
Protesters in favor of allowing same-sex marriages held rallies last week and over the weekend throughout the state against the measure.
The Burbank Fire Department evacuated about 200 to 300 people from the third, fourth and fifth floors of the office building while a Hazardous Materials Team evaluated the powder for anthrax, a lethal substance that can be ingested or inhaled.
The powder tested negative for anthrax, Irving said.
The building occupants were allowed to return to their offices about two hours later, he said.
The person who sent the letter could face federal prosecution even though the substance tested negative, FBI spokeswoman Laura Eimiller said.
“It is absolutely a crime to even send an innocuous substance through the mail,” she said.
The FBI arrested a man in Sacramento last month in connection with an anthrax hoax, Eimiller said. He sent packages containing a white powdery substance to newspapers throughout the country, she said.
The FBI responds to all incidents in which sent mail was threatening and contained a hazardous substance, Eimiller said.
VERONICA ROCHA covers public safety and the courts. She may be reached at (818) 637-3232 or by e-mail at email@example.com.