Heemeyer took about a year and a half to prepare for his rampage. In notes found by investigators after the incident, Heemeyer wrote: “It’s interesting how I never got caught. This was a part-time project over a 1½ year time period.” Heemeyer was surprised that several men who had visited the shed late the previous year did not discover the modified bulldozer, “especially with the 2,000 lb. lift fully exposed”. “Somehow their vision was clouded”, he wrote.
The machine used in the incident was a Komatsu D355A bulldozer fitted with makeshift armor plating covering the cabin, engine and parts of the tracks. In places, the vehicle’s armor was over one foot thick, consisting of concrete sandwiched between sheets of steel to make ad-hoc composite armor. This made the machine impervious to small arms fire and resistant to explosives; three external explosions and over 200 rounds of firearm ammunition fired at the bulldozer had no effect on it. National Guard units were placed on standby orders by Governor Bill Owens for possible anti-armor support.
For visibility, the bulldozer was fitted with several video cameras linked to two monitors mounted on the vehicle’s dashboard. The cameras were protected on the outside by 3-inch shields of bullet-resistant plastic. Onboard fans and an air conditioner were used to keep Heemeyer cool while driving and compressed air nozzles were fitted to blow dust away from the video cameras.
Food, water and life support were present in the almost airtight cabin. Heemeyer had no intention of leaving the cabin once he entered; the hatch was permanently sealed. Authorities speculated Heemeyer may have used a homemade crane found in his garage to lower the armor hull over the dozer and himself. “Once he tipped that lid shut, he knew he wasn’t getting out”, Daly said. Investigators searched the garage where they believe Heemeyer built the vehicle and found cement, armor and steel.