OSLO — In a remote Norwegian town north of the Arctic Circle, a cybersecurity expert with ties to WikiLeaks checked out of a hotel, dressed in khaki hiking gear and carrying heavy baggage.

That was on Aug. 20. No one has reported seeing him since.

The disappearance of Arjen Kamphuis, 46, has so far flummoxed a widening police investigation that has chased stray clues and false leads in Norway, Denmark, Germany, the Netherlands and Sweden. Norwegian police have released statements saying that they have no idea whether he was a victim of foul play, but that they “are open to all possibilities.”

The uncertainty, and Mr. Kamphuis’s links to WikiLeaks, the anti-secrecy organization that has run afoul of governments and other powerful interests, has the internet buzzing with conspiracy theories. Some suggest a kidnapping — or worse — involving Russia, the C.I.A., MI6, Islamists or the Clintons, while others ask if he intended to disappear, possibly on some secret assignment for WikiLeaks.

Feeding the speculation is that Bodo, the town where he was last seen, is home to Norway’s main military air base, the armed forces’ joint operations center is nearby, and part of the country’s cyberdefense center is hidden in a mountain outside town. Investigators have ruled out any connection between the military presence and his disappearance.

Friends say that Mr. Kamphuis, who is Dutch, was genuinely in Norway for vacation, had been there a few times before, and had plans to return to the Netherlands.

“He’s an avid hiker and mountaineer,” said Ancilla van de Leest, 33, a friend and prominent Dutch activist for online privacy rights.

On Twitter, WikiLeaks described Mr. Kamphuis as an “associate” of Julian Assange, the group’s founder. Mr. Assange has lived for years in the Ecuadorean Embassy in London to avoid arrest and what he fears would be extradition to the United States to face prosecution for publishing government secrets.

But Mr. Kamphuis’s precise relationship with WikiLeaks is unclear. Friends say he is not close to Mr. Assange, but he has reportedly offered training to WikiLeaks members relating to “Information Security for Journalists,” a handbook he co-wrote on how to shield communications and data from government surveillance.

Bodo is on a peninsula bordering one of the hundreds of ocean inlets that crease Norway’s mountainous, wooded coastline. Police have reported that before heading north, Mr. Kamphuis bought a foldable kayak from an outdoor equipment store in the Netherlands, and told the manager that he planned to paddle the fjords.

After leaving the Scandic Hotel in Bodo on Aug. 20, Mr. Kamphuis had planned on taking a 10-hour train ride south to Trondheim, Norway, Ms. van de Leest said in a telephone interview. The Dutch police have said that he was booked on an Aug. 22 flight back to the Netherlands.

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He did not make the train or the plane, but he was not reported missing until Aug. 29, when friends and colleagues in the Netherlands alerted the Dutch police.

At first, the case was not taken seriously, Ms. van der Leest complained. But as days passed and search crews in and around Bodo failed to turn up any sign of Mr. Kamphuis, the investigation intensified, spurred on by online publicity campaigns, most of them using the hashtag #FindArjen.

The search has grown into a cross-border hunt for a man who specializes in evading detection. The Norwegian National Criminal Investigation Service, Kripos, has assigned investigators in Oslo to the case, and on Tuesday, it sent four officers to Bodo.

The Dutch police searched Mr. Kamphuis’s apartment in Amsterdam, taking DNA samples from his toothbrush, Ms. van de Leest said.

Partial breakthroughs have only added to the mystery. The police revealed that 10 days after Mr. Kamphuis had disappeared in the High North, someone tried to use his cellphone near the village of Vikesa, 650 miles south of Bodo, attempting to activate a German SIM card.

Unconfirmed sightings of the missing man have been reported in Sweden and as far away as southern Denmark, in two towns on the North Sea coast, Esbjerg and Ribe. Those reports and the SIM card pulled the German, Danish and Swedish police into the investigation.

A grass-roots, crowdsourced hunt has also taken shape online, with people passing on potential clues and sightings to the police.

Some false leads have drawn the attention of amateur sleuths and the news media, including excitement a few days ago about the sighting of an abandoned tent in Lofoten, an archipelago north of Bodo that is popular with hikers, which turned out to be unrelated to Mr. Kamphuis.

Hundreds of people are reported missing in Norway each year, and although most are located within hours or days, 15 to 30 per year are not found, officials say. Kayaking and mountain climbing accidents kill a handful of people each year, according to the Norwegian Maritime Authority and the Norwegian Mountain Climber Association.

“After all this, there is no scenario that I haven’t thought about. It could literally be anything,” Ms. van de Leest said.

“He wasn’t afraid to make enemies,” she added. “He did not ‘hide his opinions under a chair,’ as we say in Holland.”