Wife of one of Russian President Vladimir Putin’s most prominent critics says her husband has again been poisoned.

Vladimir Kara-Murza was hospitalised last week with sudden organ failure and is on life support in a medically induced coma, according to his wife, Yevgenia. On Tuesday, she said doctors have diagnosed him with an “acute poisoning by an unidentified substance,” according to the The Associated Press.

In 2015, Vladimir Kara-Murza almost died as a result of sudden kidney failure. Then, he said he had been poisoned. French laboratories could not determine a specific poison but found high levels of heavy metal in his blood at the time, according to The New York Times.

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Kara-Murza was a close associate to prominent opposition leader Boris Nemtsov, who was murdered in 2015. He is a journalist and involved in the pro-democracy group Open Russia.

Members of Congress in the United States from both parties have called on President Trump and his administration to speak out on his recent hospitalisation.