Erik Menendez Off the Hook in Prison Weed Mystery

Erik Menendez is off the hook for the package of marijuana that arrived for him at his California prison ... because prison officials say he had nothing to do with it.

We broke the story ... the Menendez brother was placed in isolation at the R.J. Donovan Correctional Facility in San Diego in October after he received some weed in the mail. The shipment triggered an investigation behind prison walls.

Our sources told us the pot package was intercepted by prison staff and the big question ... was it something Erik solicited? They wanted to know if he planned on distributing or trading the drugs or keeping them for personal use.

Turns out -- it was none of the above. A rep for the California Department of Corrections and Rehabilitation tells us ... "The investigation is complete and the allegations against him were unfounded."

We're told Erik was transferred out of isolation in the Administrative Segregation Unit and back to his regular housing unit.