MOSCOW (AP) — Russian police on Thursday announced a preliminary probe into the circumstances of the sudden illness of opposition leader Alexei Navalny, who fell into a coma last week following a suspected poisoning and amid growing pressure from Western officials to investigate.

According to a statement released Thursday by a Siberian branch of Russia's Interior Ministry, investigators in Siberia have been working on “establishing all the circumstances of the incident," conducting forensic studies and collecting items “that may have probative value."

Navalny, an opposition politician and corruption investigator who is one of President Vladimir Putin’s fiercest critics, fell ill on a flight back to Moscow from Siberia last Thursday and was taken to a hospital in the Siberian city of Omsk after the plane made an emergency landing.

Over the weekend, he was transferred to the Charité hospital in Berlin, where doctors found indications of “cholinesterase inhibitors” in his system. But the hospital in the German capital hasn't yet identified a specific substance.

Found in some drugs, pesticides and chemical nerve agents, cholinesterase inhibitors act by blocking the breakdown of a key chemical in the body, acetycholine, which transmits signals between nerve cells.

His allies insist he was deliberately poisoned and say the Kremlin was behind it, accusations that officials denounced as “empty noise.”

Navalny's team submitted a request to Russia's Investigative Committee, demanding authorities launch a criminal probe on charges of an attempt on the life of a public figure, last week, but officials appeared reluctant to start an investigation.

Kremlin spokesman Dmitry Peskov said Monday he saw no grounds for a criminal case until the cause of the politician’s condition was fully established.

The Interior Ministry's statement Thursday didn't clarify when the preliminary probe — an inquiry to determine whether a criminal investigation should be launched — had started.

Navalny ally Ivan Zhdanov said that the probe should have been launched Aug. 20, right after the politician's team submitted a request. “Open a criminal case,” Zhdanov said in a tweet Thursday.

The announcement about the inquiry comes after multiple Western and European officials — including German Chancellor Angela Merkel, U.K. Prime Minister Boris Johnson and U.S. Secretary of State Mike Pompeo — called upon Russia to start a full and transparent investigation into Navalny's condition.

On Wednesday night, the politician's illness was discussed in a phone conversation between Putin and Italian Prime Minister Giuseppe Conte.

According to the Kremlin's readout of the call, Putin pointed out that “premature and unfounded accusations” were unacceptable and underscored Russia's “interest in a thorough and objective investigation of all the circumstances of the incident.”