Airiston Helmi’s seafront headquarters has a helipad and multiple surveillance cameras like Mr. Melnikov’s island, as well as a decommissioned military landing craft that has been converted into a sauna and three other vessels. Standing guard next to the main entrance of the company’s office is a fashion mannequin dressed in military fatigues with a cracked plastic head.

Its basement, according to a recent report in Iltalehti, a Finnish newspaper, contained a communications center with sophisticated equipment far beyond what an ordinary tourism or property company would need.

Thomas Willberg, a dairy farmer whose land abuts Airiston Helmi’s headquarters on the mainland, said he was asked several times by the Russian and his associates whether he would be willing to sell his cow patch. He declined.

The farmer said he met Mr. Melnikov a few times and did occasional odd jobs for him like clearing snow, but could never figure out why the Russian needed so much security equipment or what kind of business Airiston Helmi was really in.

“Finland is maybe sending a signal to our eastern neighbor that it is ready to take action if needed,” Mr. Willberg said.

Mr. Karlsson, the former construction supervisor, refused to believe that Mr. Melnikov was setting up hideaways for Russian soldiers, noting that the businessman always insisted on having large glass windows facing the sea — not a good feature to have if bullets are flying.

All the same, he conceded that he may have been naïve about Mr. Melnikov’s intentions. “He said he had fallen in love with our archipelago and could feel safe here, unlike at home in Russia. I swallowed that explanation,” Mr. Karlsson said.

“Pavel is clearly not what I thought he was,” he said. “I keep asking myself, ‘How could I have been so wrong?’”