Heemeyer had been feuding with officials and individuals in Granby, particularly over fines for violating city ordinances and a zoning dispute regarding a concrete factory constructed opposite his muffler shop that destroyed his business.
John Bauldree, a friend of Marvin's, said that Heemeyer was a fun-loving guy. Ken Heemeyer said his brother Marvin "would bend over backwards for anyone." While many people described Heemeyer as a likable person, others said he was not someone to cross. Christie Baker told the Denver Post that Heemeyer threatened her husband after he refused to pay for a faulty muffler repair.
Heemeyer bought two acres of land from the Resolution Trust Corp., the federal agency set up to handle the assets of failed savings and loan institutions. He bought the land for $42,000 subsequently agreeing to sell it to the Docheff family, which wanted the property for a concrete batch plant. The agreed upon price was $250,000 but according to Susan Docheff, he changed his mind and upped the price to $375,000 and at some later point demanded a deal worth approximately $1 million. This negotiation happened well before the rezoning proposal was heard by the town council.
As a last measure, Heemeyer petitioned the city with his neighbors and friends, but to no avail. He couldn't function without the sewer line and the cooperation of the town.
Heemeyer 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 the road. Heemeyer complained the concrete plant had left dust on, and blocked access to, his business.
Notes found by investigators after the rampage indicate that the primary motivation for Heemeyer's bulldozer rampage was his fight to stop a concrete plant from being built near his shop. The notes indicated Heemeyer held grudges over the zoning approval. "I was always willing to be reasonable until I had to be unreasonable," Heemeyer wrote. "Sometimes reasonable men must do unreasonable things."
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 last autumn did not discover the modified bulldozer, "especially with the . lift fully exposed." "Somehow their vision was clouded," he wrote.
The machine used in the incident was a Komatsu D335A 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 visibility, the bulldozer was fitted with a video camera linked to two monitors mounted on the vehicle's dashboard. 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 ever 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.
For armament the bulldozer was fitted with a .50 caliber semi-automatic Barrett M82 rifle pointing out to the rear, a semi-auto variant of the FN FNC in front, a .223 Ruger Mini-14 to the right, a 9mm Kel-Tec P-11 semi-auto pistol and a .357 magnum revolver with which he killed himself.
After the incident, the modified bulldozer came to be known as "Killdozer".
Heemeyer's rampage resulted in 13 buildings destroyed, resulting in total damages estimated at over $7 million. The bulldozer also knocked out natural gas service to City Hall and the cement plant, and damaged a truck and part of a utility service center. Despite the great damage to property, no one besides Heemeyer was killed.
According to Grand County commissioner James Newberry, Grand County emergency dispatchers used the reverse 911 emergency system to notify many residents and property owners of the rampage going on in the town. Thus, many people were forewarned and were able to get out of harm's way.
Defenders of Heemeyer contended that he made a point of not hurting anybody during his bulldozer rampage; Ian Daugherty, a bakery owner, said Heemeyer "went out of his way" not to harm anyone. Others offered different views. The sheriff's department argues that the fact that no one was injured was due more to luck than intent. Heemeyer had installed two rifles in firing ports on the inside of the bulldozer, and fired 15 bullets from his rifle at power transformers and propane tanks. "Had these tanks ruptured and exploded, anyone within one-half mile of the explosion could have been endangered," the sheriff's department said; within this range were 12 police officers and residents of a senior citizens complex. The sheriff's department also asserted Heemeyer fired many bullets from his semi-automatic rifle at Cody Docheff when Docheff tried to stop the assault on his concrete batch plant by using a front-end loader. Later, Heemeyer fired on two state troopers before they had fired at him. The sheriff's department also notes that 11 of the 13 buildings Heemeyer bulldozed were occupied until moments before their destruction. At the town library, for example, a children's program was in progress when the incident began. According to Allen Best, there might have been casualties if local emergency response, allied with a dose of luck, hadn't worked so effectively.
Two things conspired against Heemeyer as he reduced the Gambles hardware store to rubble. The radiator of the dozer had been damaged and the engine was leaking various fluids, and Gambles had a small basement. The bulldozer's engine failed and Heemeyer dropped one tread into the basement and couldn't get out. The bulldozer became stuck. About a minute later, one of the SWAT team members who had swarmed around the machine reported hearing a single gunshot from inside the sealed cab. The coroner stated that Heemeyer used his .357-caliber handgun to kill himself.
Heemeyer's body was subsequently removed by police with a crane, though it took twelve hours for them to cut through the hatch with an oxyacetylene cutting torch.
On April 19, 2005, it was announced that Heemeyer's bulldozer was being taken apart for scrap metal. It was planned that individual pieces would be dispersed to many separate scrap yards to prevent admirers of Heemeyer from taking souvenirs.
"God built me for this job," Heemeyer said in the first recording. He even said it was God's plan that he not be married or have a family so that he could be in a position to carry out such an attack. "I think God will bless me to get the machine done, to drive it, to do the stuff that I have to do," he said. "God blessed me in advance for the task that I am about to undertake. It is my duty. God has asked me to do this. It's a cross that I am going to carry and I'm carrying it in God's name.
Heemeyer's actions were apparently a political statement. In the audio tapes, he states "Because of your anger, because of your malice, because of your hate, you would not work with me. I am going to sacrifice my life, my miserable future that you gave me, to show you that what you did is wrong."
Investigators later found Heemeyer's handwritten list of targets. According to the police, it was not just a list of buildings and businesses. The list also contained the people who had sided against him in the dispute, and a local Catholic Church.