Russian opposition activist Vladimir Kara-Murza Jr., who was twice sickened in incidents he suspected were poisonings, has been detained in Moscow by police, another prominent opposition figure said Monday.

that Kara-Murza was detained Monday near his Moscow residence. It was unclear whether he had been charged.

Kara-Murza was hospitalized with poisoning symptoms twice, in 2015 and 2017. A journalist and associate of Russian opposition leader Boris Nemtsov, who was shot and killed in 2015, and oligarch-turned-dissident Mikhail Khodorkovsky, Kara-Murza nearly died from kidney failure in the first incident. He suspects he was poisoned but no cause has been determined.

He was taken to a hospital with a sudden, similar illness in 2017 and put into a medically induced coma. His wife said doctors confirmed that he was poisoned.