The lawyer for a prominent Kremlin critic who mysteriously fell ill in Moscow last year says the Russian authorities have summoned his client for questioning over the matter after he petitioned authorities to investigate it as a deliberate poisoning.

Vadim Prokhorov told RFE/RL on January 15 that a local branch of Russia's powerful Investigative Committee contacted him earlier in the day and asked to question opposition activist Vladimir Kara-Murza, Jr., who suffered a sudden, near-fatal illness in May.

Kara-Murza, 34, is a coordinator for Open Russia, a nongovernmental organization run by former Russian oil tycoon Mikhail Khodorkovsky, an opponent of President Vladimir Putin who spent over a decade in prison in Russia and now lives in Europe.

The precise cause of Kara-Murza's illness remains unclear, and the activist believes he was intentionally poisoned in response to his political activities.

His Russian doctors have previously said he was likely poisoned by toxic levels of his prescription antidepressant that may have reacted negatively with allergy medicine he was taking.

Kara-Murza and his family dismiss this theory, saying he had not changed his medication or dosage in years, and that other medicines he was taking do not interact with the antidepressant he was taking.

Kara-Murza, whose father, also named Vladimir, hosts a talk show for RFE/RL's Russian Service, filed a formal request with the federal Investigative Committee on December 11 requesting an investigation into his illness as attempted murder.

Prokhorov told RFE/RL that he was contacted by an investigator from the Investigative Committee's local branch in Moscow's Khamovniki district, which he said indicated the authorities are not taking the request for an investigation seriously.

"The fact that they gave this, in my opinion, very important matter to the lowest level possible...probably shows that they're not terribly interested in it," he said.

Prokhorov said it was likely the Investigative Committee will ultimately decide not to open a formal criminal case.

Kara-Murza, who is currently in the Washington area, where his family lives, said he welcomed the development and planned to meet with investigators when he travels to Moscow next week and "give them all the necessary information."

"However, the fact that it took them more than a month to begin the check, and that the case was transferred to the district level, points to a lack of priority," he said.

He noted that criminal cases against foes of Putin are often handled by more senior investigators. He said this shows that "politically motivated cases against Kremlin opponents are of a much greater interest to the Investigative Committee than an attempted murder of an opposition member."