Nikos Toskas, the country's public order minister, told reporters that satellite image analysis and ground inspections suggest the fires that simultaneously broke out in multiple places were the work of arsonists.
The fires broke out Monday afternoon in at least 22 different locations near seaside communities outside the capital of Athens, the government's fire service said.
In addition to the arson investigation, authorities are probing illegally constructed buildings in the forest areas that blocked escape routes when the fires erupted, Greek Defense Minister Panos Kammenos told the BBC.
The death toll from the blazes rose to at least 82 Thursday and nearly 200 people were injured.
"We have serious indications of criminal acts... lots of fires appeared in a very short period of time," Toskas said during the press conference.
He said a "suspicious find" was made in Mati, the epicenter of the devasting fires, but he would not elaborate.
"We are troubled by many factors, and there have been physical findings that are the subject of an investigation," Toskas said.
The fires erupted in the forests near the seaside towns outside Athens. Fanned by winds of up to 60 miles per hour and fueled by parched vegetation, flames quickly grew into infernos that swept across the region, leveling homes, trapping victims and prompting thousands of people to flee for their lives.
Officials said more than 1,000 homes were destroyed or damaged and at least 300 vehicles were burned.
The fires also engulfed the Lyreion orphanage near Neos Voutzas, destroying several buildings, said Valerie Kontakos, who filmed a 2013 documentary on the facility run by Greek Orthodox nuns.
"They were very lucky. Everyone is safe," Kontakos told ABC News on Thursday, after visiting the 50-year-old orphanage.
She said the more than 60 children who live at the orphanage were away at camp when the fires struck. She said that among the buildings on the property that were destroyed was one that houses elderly people, who have no family and were taken in by the nuns.
"Of the homes for the kids, one of them was destroyed and another one was damaged," Kontakos said.
She said an olive grove and vegetable garden on the property were also burned.