SOCHI, Russia — Fragments of a plane were dragged out of the Black Sea today amid a massive search operation as Russian officials declared that their investigation into the crash would focus on technical faults or pilot errors — not terrorism.

On shore, the nation held a day of mourning, laying flowers and lighting candles for the victims, who included dozens of singers in Russia’s world-famous military choir, nine Russian journalists and a Russian doctor known for her charity work in war zones.

All 84 passengers and eight crew on the Russian military’s Tu-154 plane are believed to have died Sunday morning when it crashed two minutes after taking off from the southern city of Sochi en route to Syria.

Today, more than 3,500 people on 45 ships — including 135 divers from across Russia — were sweeping a vast crash site in the Black Sea and along the shore, according to the Defense Ministry. Ten helicopters, drones and two deep-water submersibles were being used to help spot bodies and debris. The search area covered over 10 square kilometers (about 4 square miles) and was plagued by underwater currents.

Divers in the afternoon found parts of the jet a mile away from the shore and 25 meters (82) under the sea, the Emergency Situations Ministry said. Some of the debris was recovered and divers were going back into the water to search for more, it said, adding the fuselage has not been found yet.

The plane’s black boxes have not been found yet either, Emergency Situations Minister Vladimir Puchkov said. Transport Minister Maxim Sokolov said plane’s flight recorders did not have radio beacons, so locating them in the water was going to be challenging.

Speaking on television Monday, Sokolov said terrorism was not among the main theories for the crash cause, and that authorities were looking into a possible technical fault or a pilot error.

The intelligence agency FSB said echoed his comments in a later statement, saying it “has not found any signs or facts pointing to a possible terror attack or sabotage on board.” The intelligence agency said it was now focusing its probe on possibilities such as a pilot error, low quality of fuel, external objects getting in the engine or an unspecified technical fault.

Rescue teams had recovered 11 bodies as well as body fragments and flew them Monday to Moscow, where the remains will be identified.

Russia also asked Georgia’s breakaway republic of Abkhazia, which lies 7 kilometers (4.3 miles) east of the Sochi airport, to help look for plane debris or bodies.

The plane began its flight from Moscow’s military airport of Chkalovsky. The FSB insisted the plane was under its surveillance and that only two people, both FSB officers, got onboard when the jet landed in Sochi for refueling. The plane did not carry any military or dual-use cargo, the FSB said.

Still, several aviation experts noted factors that could suggest a terror attack, such as the crew’s failure to report any malfunction and the fact that debris from the plane was scattered over a wide area.

“Possible malfunctions … certainly wouldn’t have prevented the crew from reporting them,” Vitaly Andreyev, a former senior Russian air traffic controller, told RIA Novosti.

The plane was taking the Defense Ministry’s choir, the Alexandrov Ensemble, to perform at a New Year’s concert at Hemeimeem air base in Syria’s coastal province of Latakia. Despite the Syrian connection, Sokolov said the government saw no need to heighten security measures at Russian airports.

The Tu-154 is a Soviet-built three-engine airliner designed in the late 1960s. The plane that crashed Sunday was built in 1983, and underwent factory check-ups and maintenance in 2014, the Defense Ministry said.

On Monday, a nationwide day of mourning for the plane victims, red and white carnations piled up outside the Moscow office of the Alexandrov Ensemble. One singer who did not get on the plane for personal reasons said he was devastated at the loss of so many talented colleagues.

Soloist Vadim Ananyev had stayed behind to help his wife with the kids because they just had a new baby.

“I have lost my friends and colleagues, all killed, all five soloists – I feel in complete disarray,” Ananyev told The Associated Press. “It is such a shame. I have known these people for 30 years. I know their wives and children. I feel terrible for the children and for all that I have lost.”

Ananyev said he had received condolences from all over Russia and from abroad.

“We were loved all over the world, never mind the political situation,” he said.

Mourners also lit candles and brought flowers to Channel One and NTV, whose TV journalists were going to Syria to cover the concert, and to a charity founded by Dr. Yelizaveta Glinka, who was on the plane bringing medicines to Syria.

At Sochi’s brand-new Adler airport, which was built for the 2014 Winter Olympics, mourners came to light candles at the airport’s chapel and lay flowers at an improvised shrine that featured photos of some of the victims. Locals also flocked to the city’s port a few miles from the crash site to leave flowers by the waterfront.

Russian TV channels took entertainment shows off their programs Monday and outdoor seasonal celebrations were scrapped across Russia.

In Rome, Pope Francis led thousands of faithful in silent prayer for the plane crash victims and noted that the Russian army choir had performed in 2004 at the Vatican.