A bodybuilder sent Kobe Bryant a letter in 2003 offering to kill his rape accuser ... and the guy later asked for $3 MILLION to do the deed -- this according to newly released FBI docs.
The papers -- obtained by TMZ Sports -- were released by feds earlier this month ... and they reveal new details from the '03 case against Patrick Graber.
The docs, in which Graber's name is redacted, show how feds say they got Graber to fess up to attempting to get cash from Bryant in exchange for making his "problem" disappear.