One of Russian President Vladimir Putin’s fiercest critics is in the hospital after suffering sudden organ failure, the BBC reports.

Opposition activist and journalist Vladimir Kara-Murza is on life support in a medically induced coma, his wife said. She said the “clinical picture” was identical to that of an incident in 2015, when Kara-Murza almost died from sudden kidney failure.

At the time, tests found that the kidney failure had been caused by a poisonous substance, though it was unclear if Kara-Murza was intentionally poisoned or if it was an accident.

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"The reason is unclear like last time. He's been active and healthy [recently]," his wife, Evgenia Kara-Murza, told the BBC.

Some are concerned that the Kremlin critic may have been deliberately poisoned.

Vladimir Kara-Murza, a coordinator for the pro-democracy group Open Russia, was a close associate of opposition leader Boris Nemtsov, who was murdered in 2015. At the time, Nemtsov was allegedly working on a report detailing the Kremlin’s role in supplying weapons to Russian separatists in eastern Ukraine.

In a Facebook post on Wednesday, Kara-Murza posted a tribute to Nemtsov on Facebook, showing a dozen roses placed on the bridge where the opposition leader was killed. “We’re here. We remember,” the post says.

In the U.S., Sen. Ben Cardin Benjamin (Ben) Louis CardinPPP application window closes after coronavirus talks deadlock Congress eyes tighter restrictions on next round of small business help Senate passes extension of application deadline for PPP small-business loans MORE (D-Md.) called Kara-Murza's hospitalization a "test for the Trump administration," and urged President Trump and Secretary of State Rex Tillerson to speak out on the issue.

He also suggested that the activist's sudden organ failure may have been the result of a deliberate attack.

"We do not know the details on the cause for this most recent health issue, but it appears to be part of an alarming trend where Russian political opposition are targeted for their work," Cardin said in a statement.

"Vladimir is a courageous advocate for the democratic process and fundamental universal human rights," he added. "His hospitalization is a test for the Trump administration."

Sen. Marco Rubio Marco Antonio RubioThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden couldn't be more different on climate change Congress should support independent oversight of federal prisons Senate panel seeks documents in probe of DHS whistleblower complaint MORE (R-Fla.) was more direct in alleging the Kremlin's role in Kara-Murza's hospitalization, echoing Cardin's call for the Trump administration to address the matter.

"Vladimir Putin does not deserve any benefit of the doubt here, given how commonplace political assassinations and poisonings have become under his regime," he said in a statement.

Updated: 3:24 p.m.