The Russian authorities have detained three men suspected of illegally obtaining the personal data of Federal Security Service (FSB) agents whom jailed opposition leader Aleksei Navalny has accused of being involved in his poisoning last year.

The TASS news agency and the Baza news website quoted law enforcement sources on November 2 as saying the three suspects from a private investigation firm gathered the phone billing data of the FSB agents on behalf of Navalny and his associates.

"Some of the information that was obtained by the detainees was used by Navalny for one of his investigations," TASS quoted a law enforcement official as saying.

The three suspects -- Pyotr Katkov, Aleksandr Zelentsov, and Igor Zaitsev -- have been charged with forgery and violation of privacy laws.

However, law enforcement officials believe the three suspects did not know their client was Navalny, who conducted an investigation in cooperation with news outlets piecing together the movements of purported FSB agents trailing him before his near-fatal poisoning.

In December, investigative website Bellingcat, The Insider, CNN, and German magazine Der Spiegel published a report naming eight men with suspected ties to the FSB's toxins team who had allegedly taken turns trailing Navalny on trips in the three years leading up to his poisoning.

Navalny was detained at a Moscow airport in January upon his arrival from Germany, where he was recovering from a poisoning in Siberia from what several European laboratories concluded was a military-grade chemical nerve agent.

In February, a Moscow court ruled that while recovering in Germany, Navalny had violated the terms of parole from an old embezzlement case that is widely considered politically motivated. He was sentenced to 2 1/2 years in prison.

Navalny has insisted that his poisoning was ordered directly by President Vladimir Putin, which the Kremlin has denied.

With reporting by Baza and TASS