Russian authorities on Sunday arrested opposition leader Aleksei Navalny on his arrival at Moscow’s Sheremetyevo airport. Navalny was traveling from Berlin where he spent the past five months recovering from a fatal nerve-agent attack that he believes was carried out by Kremlin agents to quash his resistance movement.

Amnesty International issued a statement calling for Navalny’s immediate release and directed authorities to launch an investigation into his poisoning that occurred on August 20, 2020. Amnesty International Moscow Director Natalia Zviagina stated, “Aleksei Navalny’s arrest is further evidence that Russian authorities are seeking to silence him. His detention only highlights the need to investigate his allegations that he was poisoned by state agents acting on orders from the highest levels.”

Navalny made international headlines when he collapsed on a domestic flight and fell into a coma. After being rushed to a hospital in Omsk, a German charity organization urged Russian officials to allow Navalny to be transported to a Berlin hospital to be treated for poisoning. Weeks later, the German government determined that Navalny had been poisoned by a chemical nerve agent of the “Novichok” group — a chemical weapon developed by Russia in the 1980s and 1990s. The Kremlin has denied responsibility for poisoning Navalny, but formerly stated that it would continue to survey him due to his ties to US spies.

Authorities have arrested Navalny multiple times over the years, each time releasing him after a period of several days or weeks. Navalny is an activist, lawyer, and politician who founded Russia of the Future — an anti-corruption opposition party — and has led mass protests against Kremlin authorities. The opposition leader also ran for office in 2018 but was barred from running after a Russian court charged him with embezzlement. Navalny denies these allegations, asserting that Russian officials fabricated false charges to silence him.