Russian investigators said today the black box retrieved from Saturday's devastating plane crash that killed Poland's president, Lech Kaczynski, did not provide any evidence that the pilots had been pressured to land.

Officials said transcripts of conversations from inside the cockpit rebutted widespread theories that the Polish delegation had insisted the pilot land, despite heavy fog at Smolensk airport.

"There is no confirmation that any of the high-ranking passengers ordered the pilots to land near Smolensk. The flight recorder, whose tapes are being deciphered, did not register any pressure on crew members," a source close to the commission investigating the crash told the Russian news agency Interfax.

This week Poland's former prime minister Leszek Miller told the Guardian he thought Kaczynski may have personally contributed to the accident by insisting the pilots land in Smolensk in western Russia. Air traffic control on the ground had told the delegation to divert the plane to either Moscow or Minsk because of low visibility.

Kaczynski had been determined to reach Smolensk to attend a memorial service on the 70th anniversary of the Katyn massacre, Miller said.

"The president so wanted to be there. The pilot knew this and so they accepted the risk and in the process lost everything," the former social democratic prime minister said on Sunday.

Russian investigators said the most compelling explanation for the crash remained pilot error. They said the crew knew Smolensk's military Severny airfield was not equipped with a modern landing navigation system, unlike international airports. "The Polish side was properly informed about this well in advance," the source said.

Initial Russian reports had said that president's Tupulov plane attempted to land three times - and that on its fourth attempt it clipped a copse of trees between 500 to 700 metres short of the runway, and immediately broke up. Today, however, they confirmed there had been only one - disastrous - attempt to land. There were no survivors. Russian TV showed pictures of the upended wing and smouldering fuselage. Small fires burned in woods shrouded in fog.

Russia said it had formally handed over the black boxes to Poland. It also returned personal belongings and documents belonging to President Kaczynski, which were retrieved from the crash site.

The catastrophe also killed the president's wife, Maria, and dozens of top officials from Poland's military and clergy, as well as historians, Solidarity activists, and senior government servants.

Forensic investigators have so far identified 70 of the 96 victims.

The bodies of the Polish president and his wife are lying in state at the presidential palace in Warsaw. A state funeral will be held in Krakow on Sunday, with the US president, Barack Obama, and his Russian counterpart, Dmitry Medvedev, among the attendees.

• This article was updated on 16 April 2010. The original referred to earlier Russian reports that the Polish plane tried to land three times before its fourth and final attempt. This information has been updated to include a new Russian report on 15 April stating that there had been a single attempt.