Kamphuis, then aged 47, had not been seen since he left his hotel in the northern Norwegian town of Bodo on August 20th, 2018.
His disappearance had triggered a wave of conspiracy theories on social media, ranging from CIA and Russian involvement to a mission to carry out a secret project for WikiLeaks founder Julian Assange.
Then police discovered a kayak in the same area where a fisherman had found some of Kamphuis' personal belongings floating in the waters about 50 kilometres east of Bodø.
"The police have concluded that Mr Kamphuis most likely suffered an accident in the evening of August 20th, 2018, while kayaking in the Skjerstad Fjord a few kilometres north of the village of Rognan in Nordland County in northern Norway, and was subsequently lost at sea," the police said in a statement.
"His body has still not been recovered," they added.
WikiLeaks, which published secret US military documents and diplomatic cables in 2010, called Kamphuis' disappearance "strange".
The mystery deepened when a phone linked to Kamphuis was briefly switched on in an area near the southwestern city of Stavanger, located 1,600 kilometres from Bodø, on August 30th last year.
But on Friday, police explained that two truck drivers from eastern Europe had found the phone, a laptop and other items belonging to Kamphuis while they were fishing in the spot where his kayak was found and took the items with them.
"The police have no reason to believe that the truck drivers were involved in the disappearance of Mr Kamphuis in any other way," the police statement said.