More than 3,000 people are involved in a major recovery operation for victims' bodies and aircraft debris from the Russian military plane that crashed into the Black Sea.
A day of mourning for the 92 people on board - including members of the army's world-famous choir - has been declared by Russian President Vladimir Putin.
Russia's transport minister said investigators were looking into all possible reasons for the cause of the Christmas Day crash.
Some fragments of the jet have been found but not the fuselage as initially reported, the Russian emergency ministry said.
Here is what we know so far:
- What happened?
The Tu-154 plane belonging to the Defence Ministry crashed into the Black Sea shortly after take-off on Sunday morning from the Russian city of Sochi.
It was heading for Latakia in Syria, but disappeared from radar two minutes into the journey.
- Who was on board the plane?
A total of 84 passengers and eight crew members were on board.
Sixty four of those belonged to Russia's famous military choir, the Alexandrov Ensemble, including its leader Valery Khalilov.
The ensemble also includes a band and a dancing company.
They were travelling to a holiday concert at the Hemeimeem air base in Syria's coastal province of Latakia.
Nine Russian journalists, including a TV crew from Channel One, were also among the passengers.
A widely revered Russian charity doctor was also on board.
Yelizaveta Glinka, known as Doctor Liza in Russia, has won broad acclaim for charity work that included missions to the war zone in eastern Ukraine.
Her foundation, Spravedlivaya Pomoshch, or Just Help, says she was accompanying a shipment of medicines for a hospital in Syria.
- What caused the crash?
A pilot error or a technical fault is likely to be the cause of Sunday's plane crash, the Russian transport minister said on Monday.
Maxim Sokolov told television reporters that terrorism was not suspected.
He reiterated comments from Sunday afternoon in which he said an "entire spectrum" of possible reasons is being considered.
But several experts noted factors that could suggest a terror attack, such as the crew's failure to report any malfunction and the fact that plane debris was scattered over a wide area.
However, Russian investigators do not regard "a terrorist act" as one of the main theories for what happened, Kremlin spokesman Dmitry Peskov said.
- The recovery operation
Over 100 divers were working from 32 ships and several helicopters to search the crash site.
Drones and submersibles were also being used to help spot bodies and debris, while powerful spotlights were enabling teams to search through the night.
Fragments of the plane were soon found at a depth of 50-70 metres while the crash site was located about 1.5 km from the shore.
Rescuers had recovered 11 bodies from the crash site by Sunday evening.
Transport Minister Maxim Sokolov said fragments of other bodies were also found.
- What type of plane was it?
The Tu-154 plane is a Soviet-built three-engine airliner designed in the late 1960s.
More than 1,000 have been built, and they have been used extensively by carriers in Russia and worldwide.
In recent years, Russian airlines have replaced their Tu-154s with more modern planes, but the military and some other government agencies in Russia have continued to use them.
- What is the plane's safety record?
The plane that crashed was built in 1983, and underwent repairs in 2014, according to the Defence Ministry.
In April 2010, a Tu-154 carrying Polish president Lech Kaczynski and 95 others crashed while trying to land in bad weather at a military airport in Smolensk in western Russia, killing everyone on board.
However that was not the only fatal Tu-154 crash that has taken place.
Only a year earlier a Tu-154 belonging to Caspian Airlines crashed at high altitude while flying from Iran to Armenia when one of its engines disintegrated at high altitude, killing all 168 people on board.
In August 2006, a Tu-154 of Russia's Pulkovo Airlines crashed in Ukraine, killing all 170 people on board. The crash was blamed on an error by its pilot, who put the plane into a spin while trying to fly over a thunderstorm.
A month later, the same model skidded off the runway and caught fire while landing in the Iranian city of Mashad, killing 29 of 147 passengers. A burst tyre was believed to be the cause.
Other fatal crashes involving Tu-154 planes include:
- August 2004 - Russia's Sibir Airlines - Suicide bomber brings down plane while flying from Moscow to Sochi, killing all 46 on board.
- July 2002 - Russia's Bashkirian Airlines - Collision with cargo plane over Germany, killing all 69 on board, including 52 children.
- February 2002 - Iran's Airtour - Plane smashes into mountains near Khorramabad, killing all 119 people on board.
- October 2001 - Russia's Sibir Airlines - Flying from Tel Aviv in Israel to Novosibirsk, Russia, plane is accidentally downed over the Black Sea by a Ukrainian missile fired during an air defense exercise, killing all 78 people on board.
- July 2001 - Russia's Vladivostokavia airline - Plane crashes while landing in the Siberian city of Irkutsk, killing all 145 on board. The crash is blamed on pilot error.
- February 1999 - China Southwest Airlines - Elevator failure caused by faulty maintenance believed to cause the plane to go into an uncontrollable dive and crash on approach to Wenzhou airport, killing all 61 people on board.
- August 1998 - Cubana airline - Flight from Quito to Havana skids off runway and ploughs into a soccer field, killing 79 people.
In January 1994 a Tu-154 plane belonging to Russia's Baikal Airlines suffered engine failure and crashed into a snowy field near the town of Irkutsk, killing 126 people. The crew ignored a warning of an engine malfunction.
There were four other fatal crashes involving Tu-154 aircraft between 1994 and 1997, all of which were blamed on pilot error.
- How have Russian and Syrian leaders reacted?
Russian President Vladimir Putin has declared Monday a nationwide day of mourning.
Syrian President Bashar Assad has sent a condolence letter to his Russian counterpart.
Assad said he received news of the crash "with deep grief and sadness."
He also expressed his condolences to all the victims' families, adding that the countries are partners in the war against terrorism.