Regional police spokesman Roland Scherscher told reporters SWAT teams discovered the body after extinguishing a fire in the small cellar that was accessible only through a hidden door.
He said the corpse was so badly burned that it was impossible to identify it but police assumed it was that of the gunman, who had holed up on the farm grounds more than 24 hours before the charred remains were found.
Police "had to push against the wall to enter the room," he said. Because "the perpetrator could have been hiding behind each corner," the search took more than four hours as police cautiously combed through the two story building, said Schercher.
Police said final identification and cause of death would have to wait until forensic experts had finished their work. They said the assumption was that the suspect had laid the fire.
The gunman killed three police officers and a paramedic early Tuesday before barricading himself in a farm building near Melk, about 70 kilometers (40 miles) west of Vienna.
During the 12-hour standoff, he shot sporadically at heavily armed police. Police said he fired the last shot about an hour before helmeted officers wearing body armor and toting assault rifles piled into three Austrian army tanks and moved in. More than 100 police officers cordoned off the area.
Austrian media said the man was a suspected poacher. Police named the suspect as 55-year-old Alois Huber, owner of a trucking firm.
The killings began about 24 hours before the suspect barricaded himself, after police stopped the man on a road near a wilderness area west of Vienna where there has been frequent poaching.
The gunman fled in his car, which landed in a ditch near the central Austrian town of Annaberg, then shot and wounded a member of Austria's Cobra SWAT team at a police checkpoint, said Interior Ministry spokesman Karl-Heinz Grundboeck. The police officer later died.
An ambulance raced to the scene, but the gunman shot and killed its driver as he tried to give first aid to the wounded policeman. The gunman also shot and wounded another officer at the scene. Grundboeck said he was expected to recover.
The attacker then fatally shot another police officer at a roadblock before barricading himself in the farm building .
Later on Tuesday, another officer who had been missing was found shot dead in his patrol car near the farm, said Grundboeck. Scherscher said that officer was apparently killed after the suspect carjacked the police vehicle and took him hostage.
Scherscher said he could not confirm an Austrian media report that the suspect was shot in the abdomen by police before he sought refuge inside the farm building.
The provincial government of Lower Austria ordered black flags flown from all public buildings in honor of the victims.
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