The couple had been traveling in the Javari Valley, in the far western side of the Brazilian Amazon, before they were killed.
The boat was at a depth of about 20 meters over 65 feet, capsized with six bags of sand to make it difficult to float, at a distance of 30 meters from the right bank of the Itacoa River near the Cachoeira community, according to a report from the Civil Police.
A suspect arrested on Saturday pointed out to authorities the location of the speedboat.
Three suspects have been arrested for murders, Brazilian Federal Police confirmed on Sunday, and another five are being investigated for their alleged involvement in hiding the corpses of victims.
The police said in a statement that investigations continue to clarify all the circumstances, the reasons and those involved in the case. Phillips and Pereira disappeared while researching for a book project on conservation efforts in the region, which authorities described as complicated and dangerous, and known to harbor illegal miners, loggers, and international drug dealers. Just days prior to their disappearance, they had received death threats. Their case has drawn global attention to the dangers faced by journalists and environmental activists in Brazil. Between 2009 and 2019 more than 300 people were killed in Brazil due to land and resource conflicts in the Amazon, according to Human Rights Watch, citing figures from the Pastoral Land Commission, a non-profit affiliated with the Catholic Church. In 2020, Global Witness ranked Brazil the fourth most dangerous country for environmental activism based on documented killings of environmental defenders. Nearly three quarters of such attacks took place in the Amazon region, it said.