The cause of death for a 22-year-old man who died aboard on a Southwest flight in August has been revealed.

Rain McMillen Walmsley died from the “combined effects of 7-Hydroxymitragynine and Mitragynine,” according to the Los Angeles County Medical Examiner’s Office. His manner of the death is listed as an accident.

7-Hydroxymitragynine and Mitragynine are compounds found in Kratom, a plant supplement that has drawn growing concern due to recent overdoses.