Polish chief prosecutor Andrzej Seremet said it was clear the crew knew disaster was "inevitable" after the plane hit tree-tops.

A Russian source said there was no evidence the pilots were pushed by their VIP passengers into taking risks.

Poland's president and numerous other top figures died in Saturday's crash.

The bodies of Lech Kaczynski and his wife Maria were the first to be repatriated from Russia.

Poland, which is in national mourning all of this week, received the remains of 35 other victims on Thursday, including those of Ryszard Kaczorowski, the last leader of Poland's government-in-exile during the period of communist rule.

In all, 67 of the 96 victims have been brought back from Russia. Most of the remains, believed to be badly disfigured and burnt, were taken to Moscow for identification shortly after the crash in the western region of Smolensk.

Mr Kaczynski and his wife are to be given a state funeral in the city of Krakow on Sunday, with world leaders including US President Barack Obama set to attend.

However, Poles are divided over whether the late leader, whose popularity ratings had fallen sharply, should be interred in Krakow's Wawel Cathedral, a site traditionally reserved for Poland's kings and heroes.

'Dramatic' seconds

Russian investigators, who have been working in conjunction with their Polish counterparts, said soon after the crash that the jet's pilots had disregarded repeated warnings from Russian air traffic controllers to land at an alternative airport because of heavy fog at Smolensk.

Some have suggested the crew were under pressure to land regardless because they were carrying their passengers to a memorial at nearby Katyn, where Stalin's secret police massacred Polish military officers during World War II.

A senior Russian aviation official, Tatyana Anodina, said on Thursday that the plane only tried to land once - not up to four times as previously reported.

An unnamed source close to the investigation in Moscow also told Russia's Interfax news agency that no evidence had been found in the flight recorders that "any of the high-ranking passengers forced the pilots to land near Smolensk".

"The panel's analysis, namely data from the black boxes, shows that it was the pilot's mistake that caused the crash," the source added.

Poland's chief military prosecutor, Krzysztof Parulski, rejected the Russian report as "speculation" and warned that it was too early to draw any conclusions since the investigation was still ongoing.

Mr Seremet, for his part, told Polish radio: "The crew was aware of the inevitability of the coming catastrophe, if only due to the plane shaking after the wings hit the trees - which we are certain happened."

Col Zbigniew Rzepa, a Polish military prosecutor, said the pilots had been aware of the imminent crash as the last seconds of the voice recordings "were dramatic". He did not elaborate.

One of the three black boxes on the Soviet-built Tu-154 Polish government jet has been returned to Poland while the other two are being examined in Moscow, Polish prosecutors' spokesman Jerzy Artymiak said.

Mr Artymiak said investigators had hoped to disclose the contents of the black boxes officially on Thursday but would wait until after the weekend's funerals.

A state funeral for those killed along with the presidential couple is due to be held on Saturday.

BBC