Heavy snowfall forced workers to search the crash site, about 70 kilometers (45 miles) southeast of Moscow, on foot and on snowmobiles as they looked for human remains and clues to what caused the incident.
"We plan to carry out the main stage of the search operation in seven days because the plane debris is scattered over a very large area," Emergency Services Minister Vladimir Puchkov was quoted as saying by the Interfax news agency.
No distress signalThe Russian-made Antonov An-148 aircraft operated by Saratov Airlines disappeared from radar screens at 2:28 p.m. local time (1128 UTC) on its way to the city of Orsk in the Ural Mountains. The pilots sent no distress signal before the plane went down.
The crash killed six crew members and 65 passengers.
Authorities said they were considering human error, technical failure and weather conditions as possible explanations. Investigators are analyzing flight information from one of the plane's two data recorders that was recovered on Monday.
Latest deadly crashRussian President Vladimir Putin has offered his "profound condolences" to victims' families and canceled a trip to the southern city of Sochi to oversee the investigative effort.
The Orenburg region, where Orsk is located, declared a day of mourning on Monday.
The crash is one of Russia's deadliest in recent years. Six people died in the far east in November when their light aircraft went down, and 92 people, including the Red Army Choir, died after a military plane crashed near Sochi in December 2016.
amp/rc (AFP, Reuters, dpa)