On July 10, 2021, about 0845 mountain daylight time, a Cessna 421C airplane, N66NC, was substantially damaged when it was involved in an accident near Longmont, Colorado. The pilot and three passengers received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he performed the “before starting engine” and “starting engine” checklists and everything was normal. He performed an engine runup and then started his takeoff roll. The pilot reported that about halfway down the runway he sensed the airplane was not accelerating as fast as it should. He attempted to rotate; however, “the airplane mushed off the runway.” The airplane settled back onto the runway, then exited off the departure end of the runway. The airplane came to rest upright, and a post-crash fire ensued. The airplane sustained substantial damage to both wings and the fuselage. The airplane was equipped with a JPI EDM 760 engine monitor. The unit was retrieved from the airplane and the NVM (nonvolatile memory) downloaded. According to the engine manufacturer, the data indicated the airplane’s engines were operating consistent with each other at takeoff power until power was reduced for the rejected takeoff. Referring to the Cessna 421C Pilot’s Operating Handbook, for the conditions at the time of the accident, the airplane’s normal takeoff distance would have been about 2,700 ft to clear a 50 ft obstacle. Density altitude at the time of the accident was 7,170 ft.