The Amazonas state police force said a 17-year-old suspect has confessed to being part of a gang that shot Emma Kelty, 43, stole her electronic gear and cash, then dumped her body in a river.
The adolescent was arrested in Codajas, 240 kilometers (150 miles) from the state capital Manaus. Two more people have been arrested and a further four are still wanted, police chief Frederico Mendes in a statement.
Kelty was a head teacher in Britain and an accomplished adventurer who'd trecked unassisted to the South Pole, hiked across the United States and was now attempting to kayak the River Amazon from its source to the ocean.
A keen social media user, she had last tweeted on September 13, near the town of Coari.
She reported having met "three lovely locals," a big change from the previous day when she tweeted about encountering "50 guys in motor boats with arrows!!!"
According to police, the detained teenager said he and the other six suspects found the British traveler camping on Boieiro island, in the Coari area.
"The adolescent said the British woman was hit by two shots from a sawn-off 20-gauge shotgun, and then the offenders threw the body of the woman into the river," the statement said.
The seven assailants tried to sell two cellphones, a tablet device and a GoPro camera in local towns, the statement said.
Kelty's siblings paid tribute to their "active and determined sister" in a statement released by the UK foreign ministry.
"In a world that is today a much smaller place, the explorer in our sister found herself seeking ways to prove that challenges were achievable.
"We are extremely proud of our sister who was dearly loved by us all and her strength will be sorely missed," her family said.
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