Russian officials probing the crash of a Syria-bound military plane said Thursday there was no explosion on board, but equipment was not functioning correctly when it plunged into the Black Sea.

The Soviet-era 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.

“There was no explosion on board, I can say that for certain,” said Sergei Bainetov, head of flight safety for the Russian air force. “But an act of terror is not necessarily an explosion, so we are not discarding this version.”

Transport Minister Maxim Sokolov said the probe had established the plane was not working normally but will not issue any conclusions before January, cautioning journalists not to jump on any theories.

“It is obvious that the equipment was functioning abnormally. Why that happened is up to experts to work out,” he told reporters.

Bainetov said the air force had ground the Tu-154, which is not used by commercial airlines, “until the first conclusions” are made about the crash.

The plane was flown by an experienced pilot, who had 4,000 hours of flight experience, including 1,500 hours on the Tu-154, and analysis of the second black box, which records conversations in the cockpit, suggests he noticed something was wrong.

“Everything was going rather normally, but one phrase from the commander… suggests that an abnormal situation began to develop,” he said. The entire flight was 70 seconds long, he added, but the “abnormal” situation lasted just 10.

Russian media reported, citing a dialogue in the cockpit, that the plane’s flaps failed to retract on time, leading to the crash, but Bainetov refused to confirm this.

‘Main search’ Over

The first victim of the crash was buried at a Moscow region military cemetery on Wednesday, officials said, but most of the bodies were still underwater despite the “main phase” of the search being over.

“At this time, everything that has to do with the plane’s crash has been brought to surface” and pieces are “being laid out on the shore,” minister Sokolov said.

“We established that the plane almost entirely broke apart when it hit the water surface and the sea bottom, which, of course, complicated the search,” he said.

So far only 19 bodies and some 230 body parts have been discovered, Sokolov said.

Residents in Sochi and Moscow laid flowers at makeshift memorials to remember the choir members and popular charity worker Yelizaveta Glinka who were on the doomed flight.

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