Traces of blood found on a boat belonging to a fisherman, who has been arrested, will be tested to see whether it is human or animal, the BBC has reported.
The pair went missing while travelling by boat on a reporting trip in the Amazon rainforest on Sunday.
Mr Pereira had recently received threats over his work against illegal fishing.
Both Mr Pereira, 41, and Mr Phillips, 57, who has written about threats to the Amazon for publications such as the Guardian and the Washington Post, are hugely experienced and planned their journeys thoroughly. They reportedly had a new boat and plenty of fuel.
And she has also called on the UK authorities to put pressure on the Brazilian government.
"We want to carry on with the search," she said in a statement to the media during a gathering in front of the Brazilian embassy in London.
"We want to find out what is happening to them and we want anyone responsible for any criminal act to be brought to justice. We want a persistent deep and open investigation."
The blood traces were found on the boat of fisherman Amarildo da Costa, who has been charged with illegal possession of restricted ammunition.
Police say he was one of the last people to see Mr Pereira and Mr Phillips.
Mr da Costa is also suspected of involvement in illegal fishing for a buyer in Peru, police sources told Reuters news agency.
Six other people have been questioned in connection with the men's disappearance.
Some 250 members of the security forces have been taking part in the search, including divers and experts in jungle terrain. Two helicopters are also being used, as well as more than a dozen boats and drones.
Lead investigator Alexandre Fontes described the area as a "very complicated region" which he said could only be reached by helicopter, small plane or boat.