SOCHI, Russia — Fragments of a plane were dragged out of the Black Sea on Monday amid a massive search operation as Russian officials declared that their investigation into the crash would focus on technical faults or pilot errors — not terrorism.

On shore, the nation held a day of mourning, laying flowers and lighting candles for the victims, who included dozens of singers in Russia’s world-famous military choir, nine Russian journalists and a Russian doctor known for her charity work in war zones.

All 84 passengers and eight crew on the Russian military’s Tu-154 plane are believed to have died Sunday morning when it crashed two minutes after taking off from the southern city of Sochi en route to Syria.