The cause of Chicago rapper Juice Wrld’s death is still unknown after the Cook County Medical Examiner’s Office released its initial findings of an autopsy on Monday.

The Medical Examiner’s Office released a statement saying “additional studies are required to establish the cause and manner of death” for the 21-year-old musician, whose real name was Jarad Higgins. Additional studies include cardiac pathology, neuropathology, toxicology and histology, the office said.

Law-enforcement officials were in the process of confiscating guns and drugs from the rapper’s flight at Chicago’s Midway Airport when he suffered convulsions and went into cardiac arrest early Sunday, police told the Chicago Tribune. The rapper was briefly revived but died at a local hospital.

According to the report, officers and agents had been waiting for the rapper’s private plane from Los Angeles because they suspected contraband was on board. As they were searching the luggage, Higgins “began convulsing [and] going into a seizure,” police sources told the paper. He briefly woke up but was “incoherent” after an agent administered two doses of the anti-overdose medication Narcan, but he was pronounced dead at 3:14 a.m. local time at Advocate Christ Medical Center in Oak Lawn, Ill. Police asked the rapper’s girlfriend, who was on the flight, whether he had any medical conditions, and replied that he “takes Percocet and has a drug problem.”

The search turned up 41 “vacuum-sealed” bags of marijuana, six bottles of prescription codeine cough syrup, two 9 mm pistols, a .40-caliber pistol, a high-capacity ammunition magazine and metal-piercing bullets, police told the paper, although no drug charges have been filed. The two guards with Higgins were charged with illegally possessing the guns and ammunition.