Russian investigators have yet to determine what caused a passenger plane belonging to the Russian Defense Ministry to crash into the Black Sea only two minutes after it took off from the airport in the southwest resort town of Sochi on Sunday. All 92 passengers and crew members on board were killed in the crash. As of Tuesday 12 bodies had been recovered.

Search and rescue teams located fragments of the plane only 1.5 kilometers from the coast of Sochi and at a depth of 50 to 70 meters below the surface of the sea.

The Tupolev Tu-154 was carrying 64 members of the Alexandrov Ensemble, the official choir of the Russian military, known popularly as the Red Army Choir. They were traveling to perform at a Christmas celebration for Russian troops stationed at the Khmeimim Air Base in Latakia, Syria.

Among the others who died in the crash were nine Russian journalists, eight military officials and Elizaveta Glinka, the founder of the Russian NGO Spravedlivaya Pomoshch (Fair Aid), who was traveling to Syria to deliver medical supplies to Tishreen University Hospital in Latakia.

While investigators have not ruled out terrorism, they have so far downplayed the possibility that the plane was brought down by a bomb or missile, instead focusing on mechanical failure, low-quality fuel, or pilot error as a cause of the crash.

According to the New York Times, the plane was built in 1983 and underwent repairs in 2014. The pilot was reportedly well-experienced with the plane, clocking at least 1,900 hours flying time on Tu-154s.

The airplane’s main black box flight data recorder was recovered on Tuesday and flown to Moscow for analysis. According to a report by LifeNews, the pilot of the plane can be heard on the black box recording shouting “the flaps, damn it!” and then, “Commander, we’re going down!” Two additional flight data recorders have been located but have yet to be recovered from the sea.

A report by Interfax quoted an anonymous source reportedly close to the investigation stating that analysis of the black box data points to pilot error. “The preliminary analysis of the flight recorder is complete,” the source stated. “It leads us to conclude that that the version of the catastrophe connected to mistakes made by the pilot of the aircraft, is the main version.”

The Tu-154 is a medium range narrow body jet first designed in the mid-1960s. It was once used widely for domestic travel throughout the Soviet Union and the Russian Federation.

Russia’s chief airline, Aeroflot, retired the use of the Tu-154 in January 2010 after more than 40 years of service. While it has been phased out by commercial airlines, the plane is still used by Russian government agencies.

The Tu-154 was listed by airlineratings.com in 2013 as one of the 10 least safe passenger aircrafts still being flown. There have been at least 16 crashes and incidents involving Tu-154 aircraft in the last 15 years.

Among the most notable crashes is the April 10, 2010 crash of a Tu-154 belonging to the Polish Air Force near Smolensk. All 96 people on board, including then President of Poland Lech Kaczyński and other leading members of Poland’s government and military, were killed during an attempted landing in heavy fog. The crash was ultimately attributed to pilot error.

All Tu-154s were permanently grounded in Iran in 2011 following several high-profile incidents including the crash of Caspian Airlines Flight 7908 in July 2009, in which all 153 passengers and 15 crew died. The plane crashed approximately 16 minutes after takeoff due to a mechanical failure in one of its engines following a bird strike, sparking a fire in the tail section.

The Ukrainian military shot down a Tu-154, Siberian Airlines Flight 1812, over the Black Sea approximately 190km southwest of Sochi in October, 2001 as it was en route from Tel Aviv, Israel to Novosibirsk. Investigators determined that the plane was blown apart by an S-200 missile, killing all 78 people on board. Ukrainian officials initially denied responsibility, but later admitted that one of their missiles hit the passenger jet after it veered off course during a military exercise in Crimea.