Vladimir Pribylovsky had co-written a number of books, none of them with titles that particularly flattered Russian President Vladimir Putin. "The Putin Corporation: The Story of Russia's Secret Takeover" was one. Another was "The Age of Assassins: The Rise and Rise of Vladimir Putin: How Scary Are Russia's New Rulers?" Now, Pribylovsky has been found dead in Moscow at the age of 59, and the cause of his death remains unclear.

Pribylovsky was found dead in his apartment, his colleague and sometimes co-author Yury Felshtinsky wrote on Facebook, Radio Free Europe reported Wednesday. Russian officials had not commented on the reported death, and Felshtinsky said he had "no further information at this point."

Pribylovsky was also the president of the Panorama Information and Research Center, a think tank in Moscow, and he had written articles for the Moscow Times and the website Open Democracy. According to his biography on the Panorama Center's website, he also translated George Orwell's "Animal Farm" into Russian.

In December 2014, in an article for Open Democracy titled "Power struggles inside the Kremlin," Pribylovsky wrote that compared to the stint of his predecessor Boris Yeltsin, "the number of freedoms and their quality has decreased" under Putin. "Power has passed to a comparatively large number of ruling clans, predominantly from the Petersburg security services and Moscow financial circles," he wrote. He also questioned whether Putin had considered the economic consequences for Russia of annexing Crimea in March 2014, as well as the impact of sanctions imposed by Western countries for Russia's involvement in conflict in Ukraine.

Putin said in an interview Monday with Bild, the German daily, that the sanctions were "severely harming Russia," even as he added that Russia was "gradually stabilizing" its economy. The numbers, however, suggest a different picture. Dropping oil prices have taken their toll on the economy, which is dependent on exporting oil, natural gas and other commodities, and the value of the ruble dropped sharply Monday to more than 76 to the U.S. dollar from 74.75. Russia's GDP is expected to fall 3.8 percent in 2015.

Long before Russia's economy was hit by tumbling oil prices, Pribylovsky was questioning Putin. "Still, as yet, the aggrieved will not dare blame Putin for their problems, at least not openly," he wrote in the December 2014 Open Democracy article. "But somewhere, sitting inside the Kremlin, biding his time, sits his successor," he added.

In February 2015, Russian opposition politician Boris Nemtsov, who had been an outspoken critic of Putin, was assassinated, and the question of who killed him sparked numerous theories but no answers.

Pribylovsky was born in 1956 in Moscow, according to the Panorama Center biography. He earned a degree in Byzantine studies from Moscow State University and had worked jobs ranging from a museum guide to night watchman.