MOSCOW -- Flight recorders revealed no evidence of an explosion on board a Russian plane that crashed, killing all 92 on board, but investigators haven't ruled out a deliberate mechanical impact to down the plane, a military official said Thursday.

Russian air force Lt. Gen. Sergei Bainetov, who heads the Defense Ministry commission conducting the crash probe, said a cockpit conversation recorder contained the captain's words that indicated a "special situation" that began unfolding on board the plane.

Bainetov wouldn't elaborate on what might have led to the crash, but noted it likely was several factors.

The Tu-154 of the Russian Defense Ministry crashed into the sea early Sunday, moments after taking off in good weather from the city of Sochi.

It was carrying members of the Alexandrov Ensemble, widely known as the Red Army Choir, to a New Year's concert at a Russian military base in Syria.

Bainetov said the plane crashed 70 seconds after takeoff from an altitude of 820 feet while traveling at 224 to 230 miles per hour.

"After deciphering the first flight recorder, we have made a conclusion that there was no explosion on board," Bainetov said at a news conference.

But asked whether that means investigators have ruled out a terror attack, Bainetov said, "We aren't ruling that version yet."

"A terror attack doesn't always involve an explosion," he said. "Along with an explosion on board, there could have been some mechanical impact."

Bainetov's words appeared to contradict a statement from Russia's top domestic security and counterterrorism agency, the FSB, which has said it found "no indications or facts pointing at the possibility of a terror attack or an act of sabotage."

It said investigators were looking into whether the crash might have been caused by bad fuel, pilot error, equipment failure or objects stuck in the engines.