Heemeyer had feuded with Granby town officials, particularly over fines for violating city health ordinances after he purchased property with no sewage system. Over about eighteen months, Heemeyer had secretly armored a Komatsu D355A bulldozer with layers of steel and concrete. On June 4, 2004, Heemeyer used the bulldozer to demolish the Granby town hall, the house of a former mayor, and several other buildings. He killed himself after the bulldozer got stuck in the basement of a hardware store he was destroying.
Marvin Heemeyer was born on October 28, 1951, in South Dakota and lived in Grand Lake, Colorado, about 16 miles (26 km) away from Granby. According to a neighbor, Heemeyer moved to town more than ten years before the incident. His friends stated that he had no relatives in the Granby–Grand Lake area.
John Bauldree, a friend of Heemeyer's, said that he was a likable person. Heemeyer's brother Ken stated that he "would bend over backwards for anyone". However, while many people described Heemeyer as an affable person, local resident Christie Baker claimed that her husband was threatened by Heemeyer after refusing to pay for a disputed muffler repair. Baker said her husband later paid Heemeyer $124.
In 1992, Heemeyer purchased 2 acres (0.8 ha) of land from the Resolution Trust Corporation, the federal agency organized to handle the assets of failed savings and loan associations, for $42,000 to build a muffler shop. He subsequently agreed to sell the land to Cody Docheff to build a concrete batch plant, Mountain Park Concrete, for $250,000. According to Susan Docheff, Heemeyer changed his mind and increased the price to $375,000, then to a deal worth approximately $1 million. This negotiation happened before the rezoning proposal was heard by the town council.
In 2001, Granby's zoning commission and trustees approved the construction of the concrete plant. Heemeyer unsuccessfully appealed the decision, claiming the construction blocked access to his shop. He was subsequently fined $2,500 for not having a septic tank on the property his muffler shop occupied.
Heemeyer's bulldozer was a modified Komatsu D355A, which he referred to as the "MK Tank" in audio recordings, 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 5,000 psi (34 MPa) 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. Three external explosions and more than 200 rounds of ammunition fired at the bulldozer had no effect on it.