A lawyer for Nadia Savchenko, a Ukrainian air force officer who is on hunger strike in a Russian prison, says Russian authorities are investigating him for potential "extremist" language posted on his Twitter account.
Mark Feigin tweeted on April 1 that an investigator from Russia's Investigative Committee called him earlier in the day to inform him that the authorities were examining whether he violated Russian laws on extremism in connection with his Twitter activities.
There was no immediate confirmation from investigators that a probe is under way.
Feigin said authorities may try to remove him as Savchenko's attorney but vowed to "not back down" in his defense of the pilot.
Savchenko, 33, is charged with involvement in a mortar attack that killed two Russian journalists covering the conflict between government forces and Russian-backed separatists in eastern Ukraine.
She says she was kidnapped by separatists in June and illegally brought to Russia.
Feigin said he suspected investigators may be examining his tweets related to the February 27 slaying of Russian opposition politician Boris Nemtsov, a former deputy prime minister and one of President Vladimir Putin's harshest critics.
"But that is just a pretext," he told Russia's state-owned RIA Novosti news agency. "This is, of course, pressure in connection with the Savchenko case."
Feigin, who has represented numerous high-profile clients, including members of the dissident punk collective Pussy Riot, tweeted hours after Nemtsov's killing that responsibility for the attack likes "completely with the Kremlin."
Putin has called the assassination-style shooting of Nemtsov a "shame" and a "tragedy," and the Kremlin has vehemently denied that it had anything to do with the slaying.
In all, five suspects have been arrested in connection with Nemtsov's killing.
Russian authorities in recent years have targeted the online activities of government critics.
In January, Russian state media watchdog Roskomnadzor blocked the website of opposition figure and anticorruption blogger Aleksei Navalny after he called for people to attend an unsanctioned rally later that month.
In February, the agency accused Twitter of violating Russian laws and said the social-media site had repeatedly ignored its demands to block "extremist" content.
Putin said in October that Russia was not planning to limit access to the Internet or put it under "total control," but will need to ensure the stability and security of its Russian segment.
He added that Russia will consistently and legally close sites disseminating or promoting extremism, xenophobia, terrorism, and child pornography.
