More than 100 police and volunteers in five-day search of Mount Buffalo national park, near Porepunkah

Police suspect fugitive Dezi Freeman died only hours after he allegedly killed two officers and fled into bushland, as a major search gets under way in the Victorian high country.

There has been no proof of life recorded for Freeman since he fled from police, near the Mount Buffalo national park, about 15 minutes after he allegedly shot dead two officers – Det Sen Const Neal Thompson and Sen Const Vadim de Waart-Hottart – in the town of Porepunkah on 26 August.

Less than two hours later, police said a single gunshot was heard coming from the bushland – a report that was later corroborated by investigators.

That report led to police congregating in the region again on Monday morning as they undertook what senior officers said would be the largest Victorian manhunt in history.

“We are comfortable that we don’t believe he is here alive, however we are keeping an open mind,” Det Insp Adam Tilley said on Monday.

“We do believe strongly that he is in this area deceased.”

Tilley said that was among the three scenarios police were exploring in relation to Freeman: he died near Mount Buffalo by self-harm or misadventure; he escaped the area and was being harboured; or that he has escaped the area and has survived without help.

The first scenario was supported by reports of a gunshot at 12.29pm on 26 August near where Freeman was last seen.

The sound was reported by a member of the public, but that information had been corroborated by police, Tilley said.

Police conducted firearms testing in the region late last year in a bid to better understand where the shot may have been fired, testing which was used to inform where officers should search this week.

Drones and cadaver dogs were involved in the five-day search which started on Monday. Tilley said it was expected that more than 1.3 square km would be searched, and that the terrain was exceptionally rugged.

“It is not going on a bushwalk, terrain like this,” Tilley said.

“It is extremely challenging, it is thick, dense bush and at times you do not know where you are stepping so you have to be extremely careful.”

Thompson, 59, was just a week away from retiring when he was killed, while De Waart-Hottart, 34, was on temporary assignment to the area. A third officer was also shot.

Freeman was home with his family when police attended the property to serve a warrant in relation to alleged historical sexual offending.

Investigators in December said they had shifted their search efforts to finding the body of the self-described “sovereign citizen”. However, a five-day effort to scour the bush with cadaver dogs and drones yielded nothing.

Teams on foot also conducted line searches and cleared caves in the heavy terrain.

Detectives from Taskforce Summit, formed in October, have investigated thousands of pieces of intelligence, including numerous tip-offs from the public.

People in the area have been advised to remain vigilant and not approach the fugitive, who is considered armed and dangerous, in the event of a sighting.

Victoria police have offered a $1m reward and the possibility of indemnity for information leading to his capture – the largest reward in the state’s history for facilitating an arrest.