Investigators scoured the scene Monday after a passenger plane crashed near Moscow minutes after take-off, killing all 71 people on board, in one of Russia’s worst-ever plane crashes.

The site of the crash was enveloped in heavy snow that was waist-high in places, making it difficult to access, with emergency workers forced to reach the wreckage by foot and use snowmobiles.

Russia’s Investigative Committee said it would consider explanations for the crash including human error, technical failure and weather conditions, as the country has experienced record snowfall in recent weeks. It did not mention the possibility of terrorism.

The Antonov An-148 plane went down in the Ramensky district around 70 kilometres (44 miles) southeast of Moscow after taking off from Domodedovo airport in the Russian capital and disappearing off the radar at 2:28 pm (1128 GMT) Sunday.

“Sixty-five passengers and six crew members were on board, and all of them died,” Russia’s office of transport investigations said in a statement.

A Swiss citizen and a citizen of Azerbaijan were among the fatalities on a list released by the emergency services ministry. Three children also died including a five-year-old girl.

The flight was operated by the domestic Saratov Airlines and was headed for Orsk, a city in the Ural mountains.

Around one hundred investigators and criminologists were working at the scene, the Investigative Committee, which investigates major incidents, said Monday.

The emergency services ministry said at least one of the two black boxes had been found.

With wreckage of the plane spread over more than 30 hectares around the crash site, it will take a week to inspect the whole area, the emergency services ministry said.

More than 900 people using equipment including drones were involved in the search operation, which has been reclassified as looking for bodies rather than survivors, the ministry said.

“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 said at the scene, quoted by Interfax news agency, adding that “heavy snow” hampered searchers.

“We walked about 600 to 700 metres across a field, with snow in places waist-deep,” said Alexei Besedin, one of the first rescuers to reach the scene, quoted by the emergency services ministry.