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The five nuclear experts killed in a plane crash in northern Russia on Wednesday had helped Iran plan its nuclear reactor at Bushehr, Russian military sources have told Israeli officials.

The five experts were among 44 people killed when the RusAir Tupolev 134 went down in heavy fog on a flight from Moscow to the northern city of Petrozavodsk. Russian sources said the investigators do not suspect sabotage and are considering human error or technical failure, even though a number of Iranian scientists have already fallen victims of mysterious accidents including plane crashes.

The loss of the experts from three different companies was confirmed in a Twitter message by an adviser to Russian President Dmitry Medvedev.

It was previously reported by Russia's RIA Novosti news agency, quoting sources from Rusatom, the state nuclear energy firm.

The five experts, including leading Russian nuclear professional Andrei Trofimov, worked at Bushehr after the contract for building the plant was transferred from the German company Siemens.

Another of the dead, Sergei Ryzhov, was also a leading player in building a nuclear plant in India. The five were on their way to Petrozavodsk to attend an important meeting.

Russian sources said the loss was a heavy blow to the country's nuclear industry that would require time and effort to recover from. The five helped complete the Bushehr reactor and ensure that it could withstand an earthquake.

Russian state television reported that RusAir had switched planes at the last minute, flying the aging Tupolev instead of one of the Canadian-made planes that normally fly the route.

This change in aircraft was made without notifying the passengers - a major breach of procedure.

After the crash, Medvedev recommended that the Tupolev 134 be taken out of service as soon as possible. Russian aviation officials said the plane did not break up or explode as it hit trees in an attempt to land. It crashed on a highway.

Hydropress, a Russian company that belongs to Rusatom, is one firm involved in the Bushehr project.