UPDATE #1 8:20 PM PT -- A rep for the FBI tells TMZ ... "The L.A.P.D., L.A. City and FBI Hazardous Materials officials responded to a report of a threatening letter containing white powder received at CBS Studios in Los Angeles."
The rep continues, "Initial field screening was negative for hazardous substances but will be transported to a regional lab for further testing. A joint investigation is continuing."
Cops and other emergency responders raced to CBS Television City studio late Friday after a white powder substance was discovered by a member of the "Dancing with the Stars" staff ... and sources tell us Bristol Palin may have been the intended target.
Law enforcement sources tell us a powder substance was discovered by a member of the "Dancing" staff ... and sources connected to the show tell us a section of the studio containing "Dancing" offices was evacuated.
Multiple sources connected to the situation tell us the powder was contained in a piece of fan mail addressed to Bristol Palin.