LONDON — A British coroner said Thursday that he could not establish whether the mysterious death of Boris A. Berezovsky, a businessman and power broker from Russia, was suicide or homicide.

“I am not saying Mr. Berezovsky took his own life,” said the coroner, Peter Bedford. “I am not saying Mr. Berezovsky was unlawfully killed. What I am saying is that the burden of proof sets such a high standard, it is impossible for me to say.”

The unusual open verdict seemed to offer an aptly enigmatic epitaph for Mr. Berezovsky, a onetime mathematics professor who lived a life of intrigue and drama in Russia from the breakup of the Soviet Union, when he acquired great wealth, until 2000, when he fell afoul of President Vladimir V. Putin and fled into self-exile in London.

From here, he mounted a sustained political campaign against Mr. Putin until 2013, when he was found dead in a locked bathroom at a luxury home outside London belonging to a former wife, Galina Besharova.