On July 30, 2020, about 0938 mountain daylight time, a Cessna 337F, N337V, was substantially damaged when it was involved in an accident near Ennis, Montana. The right-front seat student pilot was fatally injured, and the left-front seat commercial pilot-in-command (PIC) and rear-seat passenger were seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 personal cross-country flight. According to the PIC, he planned a route of flight from Helena, Montana, to Jackson Hole, Wyoming. The PIC further reported that the right seat student pilot was flying the airplane about 3,000 ft above ground level. When they decided to fly over a ski resort at Big Bear, Montana, the flying right-seat student pilot turned the airplane to the southeast in the direction of mountainous terrain. The airplane subsequently entered a boxed canyon, and with a ridge now in the airplane’s flightpath that could not be cleared, the PIC took control of the airplane and attempted an evasive maneuver to exit the boxed canyon. However, the evasive maneuver, which was unsuccessful, resulted in a stall/spin and subsequent impact with remote mountainous terrain. The PIC opined that the accident should have been easily avoided by having more than sufficient altitude before entering mountainous terrain. Additionally, the PIC reported no mechanical anomalies with the airplane or engine prior to the accident that would have precluded normal operation. The rear seat passenger reported that they departed Helena at 0845 and that the “entirety of the flight was flown by the front left-seat pilot.” He further stated that the PIC was climbing in an attempt to clear a ridge when the airplane stalled and impacted terrain in a nose-down attitude. The Montana State Medical Examiner, Missoula, Montana, performed an autopsy on the student pilot and noted the cause of death as multiple blunt force injuries. Right Seat Student Pilot Medical Review Toxicology testing performed at the Federal Aviation Administration Forensic Sciences Laboratory identified 11-hydroxy-delta-9-THC [tetrahydrocannabinol] in the urine and its metabolite, carboxy-delta-9-THC was detected in the blood and urine. Levels of 11-hydroxy-delta-9-THC were 26.4 ng/mL and levels of carboxy-delta-9-THC were 3.1 ng/mL and 1225 ng/mL, respectively. Delta-9-THC (the primary psychoactive component in cannabis) was not detected. The cannabis plant contains chemicals called cannabinoids. Significant performance impairments are usually observed for at least 1-2 hours following cannabis use, and residual effects have been reported up to 24 hours. However, it is difficult to relate blood levels of delta-9-THC to effects, and there is no mechanism for relating levels in any other specimens to psychoactive effects.