The deadly wildfires broke out on Monday and hit coastal villages popular with tourists.
A "serious" piece of information indicates that "criminal acts" were behind a fire that resulted in the deaths of most of the 82 people killed in the country's worst ever wildfires, a Greek minister has said.
Deputy Citizen Protection Minister Nikos Toskas told a press conference that "there are a number of questions and I do not hide from you that a serious piece of information has led to us opening an investigation.
The fire began in Mount Pentelicus and spread to the coastal region of Mati, east of Athens, on Monday.
He also stressed that "climate conditions, extreme due to climate change," also contributed to the fire.
Meanwhile, heavy rains have led to flash flooding in the north of Athens, three days after devastating wildfires killed scores of people.
Fire services said they received at least 10 calls from motorists whose vehicles were stranded when roads became rivers after storms in the upscale districts of Maroussi and Ekali.
A civil defence spokesman said "dozens of cars were stuck on several main roads" after the early afternoon downpour, adding that traffic was gradually returning to normal.
Firefighters said around 160 residents were briefly trapped in their homes due to the flooding.
Some rain fell on the coastal region of Mati, 25 kilometres east of the capital, where more than 80 people lost their lives on Monday night when wildfires tore through homes and hotels.
The defence ministry said the army had been called in to remove debris and dig drainage channels to prevent flooding in the fire-ravaged area.
The region around Athens in November saw 16 people killed in flash floods.