Arjen Kamphuis, a Dutch cybersecurity expert who consulted the whistleblowing website, was last seen leaving a hotel in the Norwegian town of Bodo on August 20, 2018.
"The Police has concluded that Mr Kamphuis most likely suffered an accident in the evening of August 20th 2018 while kayaking in the Skjerstad Fjord … and was subsequently lost at sea," it said.
Police added that his body was never found. But they recovered a kayak with a hole in the hull and an oar as well as some of Kamphuis' personal belongings floating in the sea.
The probe was thrown off track when his cellphone was briefly switched on ten days after his disappearance. The investigation found that the device was fished out of the river by truck drivers who thought the item was discarded.