PODGORICA, Montenegro — After multiple but unproven accusations that President Vladimir V. Putin of Russia is working hard to destabilize America’s friends in Europe, a pro-Russian mercenary detained in Montenegro is slowly spilling his guts — and providing the first insider’s account of what the authorities in this tiny Balkan nation say were Russian efforts to sow mayhem.

The man, Aleksandar Sindjelic, a veteran anti-Western activist from neighboring Serbia, has become a key informant — and a suspect — in a sprawling investigation into an alleged plot orchestrated by two Russians to seize Montenegro’s Parliament building last month, kill the prime minister and install a new government hostile to NATO.

Mr. Sindjelic’s account of the events includes a visit to Moscow in September to plan the operation and details of the encrypted phones he was asked to use to avoid eavesdropping. He has not directly implicated any Russian officials but has raised questions about the links between state agencies and a murky network of Russian nationalists active in the Balkans and in eastern Ukraine.

The Montenegrin authorities say two Russians carrying passports in the names of Eduard V. Shirikov and Vladimir N. Popov commanded the botched plot. But both men, who oversaw preparations for the operation from Belgrade, the capital of Serbia, are back in Moscow, and it is unclear whether they were traveling under real or fake identities and for whom exactly they were working.