UPDATE #2 8:27 PM PT -- A rep for ABC tells TMZ the substance was determined to be talcum powder by the LAFD and adds, "Measures were taken to secure the area and ensure the safety of personnel."
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.