The driver, identified by the town manager as Marvin Heemeyer, appeared to have shot himself, said Grand County Sheriff Rod Johnson.
Officers looking through the square-foot hole cut into the metal casing could see Heemeyer's body with a gun lying nearby, Johnson said. Authorities planned to use a crane to lift the armor shell off the bulldozer, the sheriff said.
Heemeyer plowed the armor-plated bulldozer into the town hall, a former mayor's home and at least five other buildings Friday before the machine ground to a halt in the wreckage of a warehouse.
City officials said he was angry over a zoning dispute and fines for city code violations at his business in the town about 50 miles west of Denver.
Authorities detonated three explosions and fired at least 200 rounds against the heavy steel plates welded to the bulldozer, which looked like an upside down Dumpster. After the third explosion failed, officials used a cutting torch to open the square-foot hole early Saturday, county Emergency Management Director Jim Holahan said.
Holahan said Heemeyer was armed with a .50-caliber weapon but appeared to have deliberately avoided injuring anyone during the rampage, which began about 3 p.m. Friday. No other injuries were reported.
Undersheriff Glen Trainor said the dozer's armor plates consisted of two sheets of half-inch steel with a layer of concrete between them.
Grand County Commissioner Duane Dailey said Heemeyer apparently used a video camera and two monitors found inside to guide the dozer. Two guns were mounted in front and aimed through portals. Other portals were cut in the back. It was unclear how many guns Heemeyer had in the dozer.
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," Dailey said.
Investigators searched the garage where they believe Heemeyer built the vehicle and found cement, armor and steel.
Residents of this mountain tourist town of 2,200 described a bizarre scene as the bulldozer slowly crashed through buildings, trees and lampposts, with dozens of officers walking ahead or behind it, firing into the machine and shouting at townspeople to flee.
"It looked like a futuristic tank," said Rod Moore, who watched the dozer rumble past within 15 feet of his auto garage and towing company.
One officer, later identified as Trainor, was perched on top, firing shot after shot into the top and once dropping an explosive down the exhaust pipe.
"He just kept shooting," Moore said. "The dozer was still going. He threw what looked like a flash-bang down the exhaust. It didn't do a thing."
A flash-bang produces a blinding flash and earsplitting boom designed to stun a suspect.
"Gunfire was just ringing out everywhere," said Sandra Tucker, who saw the bulldozer begin the rampage from her office on Main Street. "It sounded to me like an automatic rifle, firing about every second."
Town manager Tom Hale said Heemeyer was angry after losing a zoning dispute that allowed a cement plant to be built near his muffler shop. Heemeyer also was fined $2,500 in a separate case for not having a septic tank and for other city code violations at his business, Hale said.
When he paid the fine, he enclosed a note with his check saying "Cowards," Hale said.
"We felt he was venting his frustration that he didn't get his way," Hale said of the note. "We didn't think he was going to do something like this."
Trainor said he believes Heemeyer spent months armoring the bulldozer, and investigators were looking into whether he had help.
Hale said owners of all the buildings that were damaged had some connection to Heemeyer's disputes.
The buildings included the cement plant, a utility company, a bank, a newspaper office, a hardware store and warehouse, the home of former Mayor L.R. "Dick" Thompson and the municipal building, which also housed a library.
Crumpled patrol cars and service trucks lay in the dozer's path. A pickup was folded nearly in half and had been rammed through the wall of a building.
State aid will be available to help rebuild local government buildings, and state officials will help businesses seek federal help, said Mike Beasley, director of the state Department of Local Affairs.
The scene was reminiscent of a 1998 rampage in Alma, another town in the Colorado Rockies. Authorities said Tom Leask shot a man to death, then used a town-owned front-end loader to heavily damage the town's post office, fire department, water department and town hall.