Russian dissident journalist Elena Kostyuchenko has revealed how she was traveling to Berlin by train last autumn when she was abruptly taken ill, in a case that has led German authorities to investigate a suspected poisoning attempt.

Kostyuchenko was living in exile at the time in the German capital after being warned of Russian plans to assassinate her. She was on her way back from a trip to Munich to apply for a Ukrainian visa when she suddenly found herself drenched in strange-smelling sweat and experiencing cognitive difficulties, she wrote in the Russian-language publication Meduza this week. A long and mysterious illness then ensued, from which she has yet to completely recover, the journalist said.

An initial investigation by Berlin authorities into a suspected poisoning was closed in May due to lack of evidence, but has now been reopened after further consideration.

"I can confirm that the Berlin prosecutor is investigating against an unknown perpetrator. The case is being treated as attempted murder," German public prosecutor spokesman Sebastian Büchner told CNN.

"The case was initially closed in May but reopened in July. This was due to new considerations rather than new evidence."

Kostyuchenko had been working for the independent Russian newspaper Novaya Gazeta for 17 years when Russia's invasion of Ukraine began. The paper sent her on assignment to Ukraine at the start of the war.

In March 2022, she said she was tipped off by a source in Ukrainian military reconnaissance about Russian plans to assassinate her. She was told she must leave Ukraine immediately and could not return to Russia.

Kostyuchenko eventually fled to Germany, where she rented an apartment in Berlin and began working for Meduza on September 29. After agreeing to a reporting trip to Iran for the publication, she said she was asked to submit paperwork for a Ukrainian visa before she left -- requiring her to make the trip to Munich.

In an ordeal outlined in an article written by Kostyuchenko and published in Meduza and US publication n+1, she described how she first began to sweat profusely after leaving the Ukrainian embassy on October 18, 2022.

"When I got on the train, I found my seat and immediately went to the bathroom. I wet some paper towels and started wiping myself off with them.

"I was covered in sweat. The sweat smelled strong and strange, like rotten fruit."

"I sat down and started reading the manuscript of my book. After a while, I realized that I was just reading the same paragraph over and over and couldn't move forward. My head ached."

After leaving the train, she said she struggled to remember the way to her apartment.

"I realized that I couldn't figure out how to get home. I knew that I needed to transfer to the subway, but I couldn't figure out how."

Safety concerns over book publication

Her condition deteriorated rapidly over the next days. She developed symptoms including a sharp stomach pain, nausea, shortness of breath, and swelling.

"My face started swelling. Then my fingers. I barely managed to take my rings off and could not get them back on again. My fingers looked like sausages. Then my feet started swelling," she wrote.

After managing to get a doctor's appointment 10 days after the onset of her symptoms, medical tests showed liver enzymes five times higher than normal, and blood in her urine.

She was referred to a series of doctors who failed to diagnose her condition. She was tested for viral hepatitis, which she could have picked up during her traveling, but the results came back negative.

Eventually, she was introduced to a doctor who suggested she could have been poisoned.

Berlin police were called in to check her clothes and apartment for radiation, before carrying out a "safety check" of the property. She was also questioned by a senior detective who had run the investigation into the killing of Zelimkhan Khangoshvili, a former Chechen field commander.

In a recent interview with Russian-language news outlet Current Time, Kostyuchenko said the upcoming publication of her book on Russian history has heightened her safety concerns.

"I have a book coming out in a few weeks, and the police and investigative reporters think that the release of this book could be a trigger."

Now, she says she has largely recovered from the illness, although she has been left with "no energy."

"I want to live. That's why I'm writing this," she adds.

Other dissident Russians have been targeted outside their country. Former Russian spy Sergei Skripal was poisoned alongside his daughter Yulia with a nerve agent in the English city of Salisbury in 2018. The then-UK Prime Minister Theresa May said it was "highly likely" the attack was ordered by the Russian government. The Kremlin has repeatedly denied the accusation.

Jailed Russian opposition leader Alexei Navalny also fell ill on a flight from the Siberian city of Tomsk to Moscow in 2020. The German government said he was poisoned with a chemical nerve agent from the Novichok group.