Russian investigators are hoping aircraft black boxes will explain why two Russian planes crashed almost simultaneously yesterday.
The crashes killed at least 89 people and raised fears of terrorism ahead of polls in rebel Chechnya.
The four black boxes, retrieved from two countryside sites where slabs of twisted metal, seats and clothing were scattered for miles, were shipped to Moscow late on Wednesday.
Relatives of those who died made their way to the two crash sites in southern Russia, where investigators had scoured miles of debris for clues as to why the planes destined for different cities crashed within four minutes of each other.
Prosecutor General Vladimir Ustinov said he could not rule out a terrorist act or human and technical errors at a meeting yesterday with President Vladimir Putin, who rushed to Moscow after breaking off his holiday in the Black Sea resort of Sochi.
One aircraft, a Tu-134 flying to Volgograd, went down near the town of Tula south of Moscow. Moments later and 800 kilometres away, a Tu-154 bound for Sochi crashed near Rostov-on-Don. Sibir Airlines said the pilots of the Tu-154 had triggered a hijack alert just before their plane crashed.
It had 46 passengers and crew on board. Volga-Aviaexpress, a small regional carrier which owned the Tu-134, said the crew did not report any problems on board before the plane crashed with 43 passengers and crew.