Over the last year, the case seemed to stump investigators, who struggled to piece together his visit to Washington. The F.B.I. ultimately assisted the investigation, but officials almost from the start seemed to play down the possibility of foul play, either local or foreign.

Even now, the ruling might not end the speculation about the case.

“Given the murders of a number of Putin critics and others who know the inner workings of the Kremlin, like Lesin, it’s hard to rule out foul play in his death,” said David J. Kramer, director of human rights at the McCain Institute for International Leadership in Washington, who closely monitors Russia. “But that says more about the regime in Moscow than about D.C. authorities’ conclusions in this case.”

The state news media in Russia at the time swiftly declared Mr. Lesin’s death a heart attack, but his acquaintances also noted his heavy drinking. A business partner, Sergei A. Vasilyev, told the newspaper Kommersant shortly after his death that Mr. Lesin had met friends from Russia who lived in Washington and had been drinking heavily.

Mr. Vasilyev said Mr. Lesin would sometimes behave recklessly when intoxicated, including instances when “he fell and caused involuntary injury to himself, including quite heavily,” a description that coincides with the authorities’ latest finding.

Mr. Lesin became the minister of the press in 1999, in the twilight of Boris N. Yeltsin’s presidency, and became an instrumental player in Mr. Putin’s efforts during his first term as president to wrest control of national television networks from the tycoons who ran them.

Mr. Lesin went on to serve as a senior presidential adviser in the Kremlin and in 2005 started Russia Today, the country’s first all-news television network broadcasting in English. Known today as RT, the network has become an important weapon in the information war the Kremlin believes it is fighting against hostile governments and the news media in the West.

Mr. Lesin left government for a time, but after Mr. Putin assumed the presidency for a contentious third term in 2012, he returned to run the media arm of the state-run natural gas monopoly, Gazprom, which was controlled by one of Mr. Putin’s confidants, Yuri V. Kovalchuk. Mr. Kovalchuk and his Bank Rossiya each face sanctions by the United States for ties to Mr. Putin.