Khloe Kardashian -- Suspicious White Powder Investigated by FBI

A letter containing a suspicious white powder -- addressed to Khloe Kardashian -- was sent to CBS today where "X Factor" is taped and the FBI is investigating ... TMZ has learned.

Sources tell us ... the letter arrived to the studio this 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.