Doctors in the hospital where Vladimir Kara-Murza, 35, is being treated say they still "have no real idea" what caused his illness, but agree a "toxic substance" is to blame, his lawyer Vadim Prokhorov told CNN.
Kara-Murza's wife, Yevgenia, told CNN that doctors have tried to bring her husband out of a medically-induced coma but that when they did so, he "had almost no reaction at all, except to his name being called." He is currently on dialysis and artificial ventilation.
She said doctors have diagnosed him with "acute intoxication by an unidentified substance" and that there is "no prognosis," adding that the risks are now more serious due to his previous illness.
Prokhorov told CNN that an investigator from the Khamovniki district of Moscow contacted him last Friday to inquire about the circumstances which led to Kara-Murza's hospitalization, but has not received any further updates about the ongoing investigation.
"The problem is that they are not paying any real attention to this," Prokhorov said. "The investigative committee is only interested in this situation because of the media coverage."
He said he has no direct proof his client was poisoned. The Kremlin has denied any involvement in Kara-Murza's 2015 illness.
Moscow denies involvement in 2015 health scare
CNN spoke to Kara-Murza in 2015, months after a mysterious illness nearly killed him.
"I fell into a coma, all of my major organs began failing, one after another. It was the kidneys first, then the lungs, the heart, the liver," he said at the time.
He said he believed his 2015 suspected poisoning was a politically motivated attack.
"Frankly there is no other possible reason," he said. "I don't have any money dealings. I don't have any personal enemies. I didn't steal anybody's wife."
The case had parallels with the killing of former Russian agent Alexander Litvinenko
A British public inquiry into that 2006 poisoning found two former Russian agents responsible for carrying out the poisoning with the radioactive isotope Polonium-210. Its final report concluded that Putin was "probably" aware of the operation.
The Russian Foreign Ministry dismissed the UK inquiry as politically motivated and the Kremlin said Russia wasn't involved.
In February 2015, a leading Russian opposition figure and friend of Kara-Murza, Boris Nemtsov, was shot to death on a bridge near the Kremlin as he walked home from a restaurant. Kara-Murza became ill four months later.
US lawmakers urge investigation
Kara-Murza's hospitalization has triggered an outcry from US lawmakers, who have called on the Trump administration to speak out on his behalf. US Sen. Marco Rubio, R-Florida, said Putin should be held accountable if there is evidence the government poisoned Kara-Murza.
"Vladimir Putin does not deserve any benefit of the doubt here, given how commonplace political assassinations and poisonings have become under his regime," said Rubio, a frequent Putin critic. "I am praying that Kara-Murza's condition improves."
He urged the White House and the State Department to question Russian authorities about the case.
Kara-Murza, who splits his time between Russia and his family home in Virginia, is a dual Russian-British citizen. He is part of the Open Russia foundation, an organization of anti-Putin activists who call for open elections, a free press and civil rights reforms. When he fell ill last week, Kara-Murza was touring Russia for screenings of a documentary about Nemtsov.