“Security camera footage and the chip attached to the swimmer’s foot showed he began the race but did not get out at the finish line,” it said.
There was still no trace of him, with Istanbul prosecutors opening an investigation into his disappearance, the private DHA news agency reported.
Russia’s consulate general in Istanbul said it was in contact with Turkish authorities about the situation and confirmed Svechnikov’s relatives had been informed.
According to the official race website, swimmers have a time limit of two hours to complete the course, with the coast guard picking up anyone who does not finish within that time.
The Olympic Committee said the race began at 10.04am with the last swimmer entering the water at 10.54am.
After the two-hour race limit finished, an initial check found that three swimmers were missing, but this was later reduced to a single athlete, it said.
“This information was immediately shared with the coast guard command, the maritime police department, and the Istanbul governor’s office, and search and rescue operations were ... immediately initiated,” it said.
“We are working in full co-operation with the coast guard and police units to shed light on the incident.”
-Agence France-Presse