This is said in the report on the results of the joint investigation, published on the Bellingcat website.
"The investigation has identified, to a high degree of certainty, the identity of “Delfin,” a key figure sought by the Joint Investigation Team, as Colonel General Nikolai Fedorovich Tkachev, currently serving as the Chief Inspector of the Central Military District of the Russian Federation," the report says.
On 28 September 2016, the Dutch-led Joint Investigation Team (JIT), the criminal investigation into the downing of Malaysian Airlines Flight 17 (MH17), published a call for witnesses regarding two key individuals in the downing of the airliner: “Delfin” and “Orion,” with the first name and patronymics of (respectively) Nikolai Fedorovich and Andrei Ivanovich. In their call for witnesses, the JIT also published a number of intercepted telephone calls between these two individuals, where they address each other by their first names and patronymic.
“Over a year after the JIT’s call for witnesses, a long-running investigation that combines open source research, investigative journalism, and two independent forensic analyses has determined Delfin’s identity: Colonel General Nikolai Fedorovich Tkachev, currently serving as the Chief Inspector of the Central Military District of the Russian Federation,” reads the reports.
After concluding that Colonel General Tkachev was by far the most likely candidate to be Delfin, Bellingcat and The Insider sought samples of Tkachev’s voice to cross-reference it against Delfin’s voice in the JIT’s call for witnesses.The experts of Bellingcat and The Insider obtained samples of the voice of Nikolai Fedorovich Tkachev by conducting two telephone conversations with him. Afterwards, Bellingcat and The Insider reached out to two organizations to perform forensic voice analysis: the National Center for Media Forensics at the University of Colorado at Denver (UC Denver) and the Forensic Science Centre of Lithuania (FSCL) The analysis results showed highly probable match between the voice in the JIT’s call for witnesses and telephone conversations.
Per his official biography, Tkachev took part in both the first (1994-1996) and second (1999-2000) Chechen wars. In 2011, Colonel General Tkachev became Russia’s Head Military Advisor in Syria. As the experts believe, in 2014 Tkachev was sent to the militant-controlled territory in Luhansk region where he attempted to organize and command military formations in the “Luhansk People’s Republic”, using the call sign Delfin.