Marvin John Heemeyer (October 28, 1951 – June 4, 2004) was an American welder and an automobile muffler repair shop owner most known for his rampage with a modified bulldozer. Outraged over the outcome of a zoning dispute, he armored a Komatsu D355A bulldozer with layers of steel and concrete and used it on June 4, 2004 to demolish the town hall, the former mayor's house, and other buildings in Granby, Colorado. The rampage ended when the bulldozer got stuck in the basement of a Gambles store he was in the process of destroying. Heemeyer then killed himself with a handgun.

Heemeyer leased his business to a trash company and sold the property several months before the rampage; he had bought a bulldozer two years before the incident, with the intention of using it to build an alternative route to his muffler shop, but city officials rejected his request to build it.

Notes found by investigators after the incident indicate that the primary motivation for the bulldozer rampage was his fight to stop a concrete plant from being built near his shop. These notes indicated that he held grudges over the zoning approval. "I was always willing to be reasonable until I had to be unreasonable", he wrote. "Sometimes reasonable men must do unreasonable things." Heemeyer took about a year and a half to prepare; in his notes he wrote: "It is interesting to observe that I was never caught. This was a part-time project over a 1½ year time period." Clearly he was surprised that several men, who had visited the shed late the previous year, had not noticed the modified bulldozer "...especially with the 2000-pound lift fully exposed". "Somehow their vision was clouded", he wrote.

The machine used in the incident was a Komatsu D355A bulldozer[9] fitted with makeshift armor plating covering the cabin, engine and parts of the tracks. In places this armor was over 1 foot (30 cm) thick, consisting of 5000-psi Quikrete concrete mix sandwiched between sheets of tool steel (acquired from an automotive dealer in Denver), to make ad-hoc composite armor. This made the machine impervious to small arms fire and resistant to explosives: indeed three external explosions and more than 200 rounds of ammunition fired at the bulldozer had no effect on it.

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 (76 mm) 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. He had made three gun-ports, fitted for a .50 caliber sniper rifle, a .308 semi-automatic, and a .22 long rifle, all fitted with a half-inch-thick steel plate. Heemeyer apparently had no intention of leaving the cabin once he entered it. Authorities speculated he 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 and armor steel.

Only Heemeyer was killed in the event (by a self-inflicted gun shot) but afterwards the modified bulldozer came to be known as "Killdozer" (similar to the name of a story written by Theodore Sturgeon).