Tajik dissident Umarali Kuvatov was poisoned before he was shot dead in Istanbul in late March, a forensic report said Wednesday.
The Istanbul-based Council of Forensic Medicine said in its report, which was also sent to Istanbul's prosecutor's office, that the drug clozapine was traced in Kuvatov's blood.
Although the dose was not determined to be lethal, forensic experts believe the drug, which is normally used to treat schizophrenia, had been given to Kuvatov with the aim of poisoning him.
Kuvatov belonged to Tajikistan's Group 24 opposition movement. His wife Kumrinisso blamed the Tajikistan regime for Kuvatov's death.
The Supreme Court of Tajikistan has labeled Group 24 an "extremist" organization and banned its activities in October 2014 after the opposition group called for anti-government protests.
Tajikistan's chief prosecutor's office claims the Group 24's objective is to seize power in an unconstitutional way, a charge the movement said was "politically motivated."
A hub of migration for people from former Soviet Central Asian republics and the Caucasus region, Turkey has witnessed a string of murders targeting activists from those countries, largely blamed on Russia and hardline regimes in their countries.
In December, Abdullah Bukhari, an Uzbek preacher known as a prominent dissident against the Karimov regime, was killed in broad daylight by a Chechen suspect in Istanbul's Zeytinburnu district. Zeytinburnu also saw a string of murders between 2008 and 2011 targeting Chechen dissidents. Six figures from the Chechen opposition were killed in attacks blamed on Russian intelligence. In February, Kaim Saduev, a Chechen dissident, died of food poisoning in Istanbul after eating food sent from Chechnya. His family has claimed the food was laced with poison during its delivery to Turkey.