Craig Ferguson -- Cops Investigating Anthrax Scare

Cops raced to CBS Television City in Hollywood yesterday -- after talk show host Craig Ferguson received a letter in the mail containing some serious threats AND an unknown white powder.

Ferguson says the person who wrote the letter claimed the substance was anthrax -- and both of the CBS staffers who were exposed to the letter were quarantined while officials conducted an investigation.

According to the AP, detectives from the LAPD's Major Crimes Unit were working with federal and European agents to find out who sent the letter.

Law enforcement sources tell TMZ ... the substance was determined to be cornmeal.

Ferguson has since tweeted about the situation ... saying, "Ack! Someone mailed my show white powder & claimed it was anthrax. I'm not a big fan of that sort of thing."

Last night, Craig joked about the situation on the "Late Late Show."