It is “highly plausible” that a member of Russia’s Pussy Riot activist group was poisoned in Moscow last week, German doctors who have been treating him said Tuesday.

Doctors at Berlin’s Charite hospital said the rapid onset of symptoms was consistent with a disruption of the nervous system, although they have not been able to determine the substance involved.

“We have no indication whatsoever that an endogenous disease or infection or some sort of metabolic disease can cause these types of symptoms,” said the hospital’s chairman, Karl Max Einhäupl. “And that’s why we have to assume that an external toxin, which we haven’t yet identified and maybe won’t identify, was introduced.”

Pyotr Verzilov, a 30-year-old Russian-Canadian citizen and prominent figure in Russia’s anti-Kremlin movement, remains in intensive care but is “no longer in life-threatening danger,” Einhäupl said.

Verzilov attended a friend’s court hearing in Moscow on Sept. 11 and soon after began losing vision, speech, mobility and then consciousness.

He received initial treatment in a Moscow intensive care unit and toxicology department, where Russian doctors pumped his stomach and said the symptoms were caused by a “pre-stroke condition,” according to Russia’s Interfax news agency.

But friends and family feared poisoning and so, with the help of a nonprofit humanitarian group, flew him to Berlin.

“It was clear to me from the beginning that we’re dealing with an attempt at intimidation, at the least, but probably with attempted murder,” Nadya Tolokonnikova, Verzilov’s former partner and fellow Pussy Riot member, told the German daily Spiegel Online.

The Kremlin has not commented on the allegation, which comes as relations between the West and Russia are at Cold War-era lows over a range of issues — including election interference and the March poisoning of former Russian spy Sergei Skripal and daughter Yulia Skripal in the British city of Salisbury. Britain has blamed Russian intelligence agents for the Novichok nerve agent attack and, along with Western allies, expelled dozens of diplomats. Russia denies any involvement.

The Pussy Riot art collective has been a thorn in Russian President Vladimir Putin’s side since it came together just under a decade ago. Its members have routinely criticized Putin.

Verzilov and three others served 15-day jail sentences for disrupting July’s World Cup final to protest Russian police powers. Verzilov is also publisher of ­MediaZona, an independent news website that features investigations of Russia’s criminal justice system.

The Berlin doctors say that it’s not clear how long he will need to remain in the hospital but that they are “confident that he will fully recover.” Tolokonnikova told Spiegel Online she thinks he will want to return to Russia as quickly as possible, despite the dangers he may face upon returning.

Amie Ferris-Rotman in Moscow contributed to this report.