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The private jet Juice WRLD flew on to Chicago was loaded down with 70 pounds of marijuana ... and cops say people on the plane told them the rapper had popped several pills prior to suffering seizures.

Law enforcement sources tell TMZ ... the FBI confiscated the huge stash of weed from several suitcases that were on board the flight. We obtained this photo of the marijuana and luggage in the private jet terminal at Midway.

According to law enforcement, several members of Juice's team aboard the flight -- which landed around 1 AM Sunday -- told them Juice had taken "several unknown pills" prior to the seizures which led to him getting rushed to a hospital.

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Further, we're told one of Juice's associates told authorities the 21-year-old rapper has a Percocet problem. Our law enforcement sources also say a bottle of codeine cough syrup was found on the plane.

Juice has admitted abusing lean, or codeine cough syrup, in the past -- and taking prescription pills like Percocet. His cause of death hasn't been determined ... the Cook County Medical Examiner will conduct an autopsy on Monday.

Two men -- Chris Long and Henry Dean -- were arrested for possession of handguns. We're told cops recovered 3 guns in total. Dean was released without bail, and Long is out after posting a $1,500 bond.