FBI Investigating White Powder Sent To Khloe Kardashian

Sources tell us ... the letter arrived to the studio yesterday morning ... and we're told the FBI arrived to inspect the mystery substance. Sources tell us ... law enforcement determined the powder was harmless.

We're told a letter was also inside the envelope ... we do not know what was written. This is the 3rd instance we know of where a suspicious white powder was sent to the CBS studio -- two letters were sent to Bristol Palin, and just as in this case the white substance turned out to be harmless talcum powder.