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.

Federal investigators in the docs say Graber — who they say was a gym rat — mailed a FedEx letter to Bryant with the offer … adding that Bryant’s team immediately turned it over to local authorities.

In the docs, officials say they then set up meetings with Graber … and he allegedly told them the price tag he wanted in order to make Kobe’s “problem” go away.

Investigators in the docs say deputies traveled to Kobe’s accuser’s residence in Colorado to tell her of the possible threat.

Graber was eventually arrested after a lengthy investigation … and was hit with several charges, including solicitation to commit murder.

Graber later pled no contest to grand theft … and was ultimately sentenced to three years in prison.