It is believed the two men, whose dead bodies were found on farmland on Saturday, had unwittingly ingested a poisonous substance during their time outdoors, which resulted in their deaths.
Ricardo Marques, 35, who was in the Air Force, and Filipe Namora, 43, are suspected of having eaten a poisonous plant or a plant covered in pesticide.
An autopsy scheduled for this week was expected to confirm the exact cause of the deaths.
Hours before their bodies were found, Filipe Namora wrote on his Facebook page that they had eaten fennel and wild carrots.
Fears are that they may have mistakenly eaten a similar-looking, poisonous species of plant, or ingested vegetables or fruits treated with pesticides.
Newspaper, Correio da Manhã, reported the friends had trekked around nine kilometres before the younger man complained of feeling unwell.
Filipe Namora called emergency services to ask for help, telling the local fire service his friend couldn’t walk and felt ill, but only managed to explain that they were located a few kilometres from Pernes, near a water conduct. He too then became unwell.
The tragic duo was found some 40 minutes from the spot of their last known point of communication.
Bottles of water and a backpack with camping equipment were found near their bodies.