On August 31, 2019, about 1105 Pacific daylight time, a Cessna 510 airplane, N551WH, was substantially damaged when it was involved in an accident near El Monte, California. The airline transport pilot and passenger sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, while holding short of the runway while waiting for his takeoff clearance, he set the parking brake. Upon obtaining his takeoff clearance, he reached down to the parking brake handle and, "quickly pushed the parking brakes back in." During the takeoff roll, he noticed that the airplane was not accelerating beyond about 70 knots and decided to abort the takeoff. The airplane subsequently veered to the left, exited the runway, impacted a fence, and came to rest upright about 800 ft beyond the departure end of the runway. The pilot stated that he must not have pushed the parking brake handle all the way in, and that he never visually verified its position before takeoff. The pilot further reported that there were no mechanical issues with the airplane that would have precluded normal operation. Examination of the airplane by a Federal Aviation Administration inspector revealed that the airplane came to rest upright at the airport perimeter fence. The fuselage and both wings were structurally damaged. The inspector observed the parking brake handle partially extended, and when he pulled the handle, it moved about 1/2-inch.