Three years ago, in February 2017, Vladimir Kara-Murza was rushed to a Moscow hospital, where he suffered massive organ failure, forcing doctors to place the Russian democracy activist on a ventilator, put him in a coma, and purify his blood. The symptoms were almost identical to what had happened to him two years earlier.

Days after the second incident, Kara-Murza’s wife took some of the blood drawn by the Moscow doctors and flew to the United States. Upon arrival, she was met by FBI agents who took the samples for testing. After U.S. senators, including Marco Rubio (Republican-Florida) and Roger Wicker (Republican-Mississippi), took a specific interest in the case, FBI officials assured another senator, the late John McCain (Republican-Arizona), that some sort of result would be forthcoming.

But the FBI then reversed itself, according to congressional officials, and declined to release any results: not to Kara-Murza, and not to Congress.

In a lawsuit being filed February 25 in U.S. District Court in Washington, D.C., Kara-Murza is suing the FBI to obtain records and results of the tests, charging that the U.S. Justice Department -- of which the FBI is a part -- was improperly withholding them.

“Vladimir wants to know what they found,” Kara-Murza’s lawyer, Stephen Rademaker, told RFE/RL. “Any person who was poisoned wants to know what he’s been poisoned with.”

The FBI did not immediately respond to inquiries from RFE/RL, which has also sought access to the agency’s toxicology records and other test results.

In a statement to RFE/RL, Kara-Murza said he believed the FBI records, if they prove he was poisoned, would help protect him against any future threats in Russia.

“I need this not out of curiosity or for vain interest, but as a small measure of protection against repeated attacks on my life,” Kara-Murza said.

The lawsuit comes amid heightened concern about the activities of Russian security and intelligence agencies and their operations in Europe and elsewhere. The poisonings of former Russian spy Sergei Skripal in England in March 2018 and a Bulgarian arms dealer in Sofia in 2015 focused attention on a unit operated by Russian military intelligence that is known for conducting sabotage, surveillance, and even assassinations outside of Russia. Aleksandr Litvinenko, a former Russian intelligence officer, died in London in 2006 after being poisoned with a highly radioactive isotope.

Within Russia, there are prominent cases of Russians who have fallen ill under suspicious circumstances -- for example, Yury Shchekochikhin, a journalist and liberal lawmaker who died in 2003 following a weeks-long battle with a mysterious illness that his colleagues believe was a deliberate poisoning.

The legal action in U.S. federal court also coincides with the fifth anniversary of the killing of Kara-Murza’s friend and political mentor, Boris Nemtsov, who was gunned down on a bridge near the Kremlin walls on February 27, 2015. A former deputy prime minister, Nemtsov was an outspoken opponent of President Vladimir Putin and was seen by many opposition activists as a potential rival to Putin.

Five men from Russia’s Chechnya region have been convicted and sentenced to prison for Nemtsov’s killing, but his relatives and supporters say justice will not be done until those behind the slaying are identified and prosecuted.

In the months immediately following Nemtsov’s killing, Kara-Murza, who owns a home in the United States, continued traveling to Russia, and elsewhere, to raise awareness of the Nemtsov murder investigation and to highlight growing repression in Russia under Putin.

Kara-Murza had also worked for Open Russia, a group funded by Mikhail Khodorkovsky, the exiled oil tycoon who is also a vocal Kremlin opponent. In the United States, he also actively lobbied for a law passed in 2012 called the Magnitsky Act, which sanctioned Russians deemed to have committed human rights abuses or financial crimes.

On February 2, 2017, Kara-Murza fell ill in Moscow and was hospitalized in an intensive-care unit for weeks.

Doctors initially concluded that he had suffered acute "toxicity from an unspecified substance,” according to his wife, Yevgenia.

In the previous medical emergency, in 2015, his Russian doctors had suggested he may have suffered unintentional poisoning due to a widely prescribed antidepressant he had taken, though Kara-Murza and his supporters dismissed that possibility. Independent toxicologists also called this scenario highly unlikely. He stopped taking the antidepressant after the 2015 incident.

On February 19, 2017, nearly three weeks after the second hospitalization, Yevgenia brought blood samples to the United States for testing, and also provided samples to laboratories in France and Israel.

Neither of the laboratories that tested the samples on behalf of Kara-Murza reached a definite conclusion on the cause of his illness.

Kara-Murza says he believes his illness was caused by a sophisticated toxin that would likely only be accessible to security agencies. In July 2018, he formally requested records and results of the FBI’s analysis under U.S. Freedom of Information Act (FOIA) and Privacy Act laws.

The FBI, according to the lawsuit, has stated it has relevant records, but also has denied his request to release the records more quickly, despite provisions in the law that allow for expedited processing “in cases involving threats to the life or physical safety of an individual.”

In Russia, Kara-Murza’s lawyer, Vadim Prokhorov, has filed appeals to the Investigative Committee – the equivalent of the FBI -- for help in investigating both incidents. To date, there has been no response, he told RFE/RL.

“Just as so many of my colleagues in the pro-democracy opposition, I know the risks our work involves,” Kara-Murza said. “We do what we do because we believe in our cause -- and in our country. And if there is any way of defending ourselves, it is with transparency and truth. This is what my lawsuit aims to achieve.”

The offices of U.S. senators Rubio or Wicker did not respond to queries about the lawsuit or whether they had concerns about the FBI’s administrative procedures.