Norwegian police have launched an investigation into the "strange disappearance" of Arjen Kamphuis - a Dutch citizen and associate of WikiLeaks founder Julian Assange.

Kamphuis, an avid hiker, disappeared after leaving his hotel in Bodø, Norway on August 20 according to the WikiLeaks Twitter account. He was set to fly out of Trondheim on August 22 after a 10-hour train ride, "suggesting that he disappeared in within hours in Bodø, Trondheim or on the train."

Update on the strange disappearance of @ArjenKamphuis. Arjen left his hotel in Bodø on August 20. He had a ticket flying out of Trondheim on August 22. The train between the two takes ~10 hours, suggesting that he disappeared in within hours in Bodø, Trondheim or on the train. https://t.co/t4OTJZGBeT — WikiLeaks (@wikileaks) September 2, 2018

A website set up to gather information on the missing person says: "He is 47 years old, 1.78 meters tall and has a normal posture. He was usually dressed in black and carrying his black backpack. He is an avid hiker." There have reportedly been two unconfirmed possible sightings, one in Alesund, Norway, and the other in Ribe, Denmark. -DW

Norwegian police began investigating Kamphuis's disappearance on Sunday.

"We have started an investigation," police spokesman Tommy Bech told AFP, adding that so far they have "no clue" where the Dutch citizen is and "would not speculate about what may have happened to him."

Until Kamphuis is officially reported missing in the Netherlands, Norwegian police cannot legally access his cellphone's tracking data, according to the Norwegian Verdens Gang tabloid. The Dutch Ministry of Foreign Affairs, meanwhile, said that it is aware of the disappearance after a friend of Kamphuis and privacy activist Ancilla van de Leest sent a tweet about his disappearance.

The Norwegian news agency, NRK had this short update at 11.05 today, 2/9. It says: 'Still no trace of Arjan Kamphuis.There is still no trace of the Dutch male that is reported missing in Bodø,says the police.He was last seen when he checked out of a hotel [in bodø] 20th August. pic.twitter.com/UY89hr0soa — Øystein L. Andersen (@OysteinLAnderse) September 2, 2018

The first report of Kamphuis's disappearance was reported by De Leest on Friday, writing that Kamphuis was set to board a train from Bodø to Trondheim.

A train trip Bodø-Trondheim takes apprx 10hrs pic.twitter.com/Zd67XntyxC — Ancilla (@ncilla) September 1, 2018

Speculation over what's happened to the Assange associate has ranged from "going dark" for a clandestine WikiLeaks mission, to killed by government operatives.

Could He Be On A "Classified" Or "Undercover Assignment" ? pic.twitter.com/RY1uljuh8x — System_On_Chip (@US_Officials) September 2, 2018

Captured or killed by agents of the five eyes alliance? — Bjornar Simonsen (@BjornarF1) September 2, 2018

Pretty loud message to Assange...flip and you will get murdered. — Bossanova (@Bossanova98) September 2, 2018