A security source told the RIA Novosti news agency that human error was most likely to blame.

"A mistake by the crew during landing maneuvers has supposedly caused the crash," the source said.

The Interfax news agency cited the deputy head of Russian air force headquarters as saying air traffic controllers had advised the pilot not to land at the airport due to heavy fog, but the suggestion was ignored.

In a television address to the Polish people on Saturday, Russian President Dmitry Medvedev announced April 12 a day of national mourning for the victims of the plane crash.

Medvedev stressed that he had ordered a thorough investigation of the causes of the crash.

"This work will be done in close interaction with corresponding Polish structures and agencies," he said. "All instructions have been given to this end."

Earlier in the day, Medvedev sent Minister for Emergency Situations Sergei Shoigu to the crash site and ordered the establishment of an investigation committee headed by Prime Minister Vladimir Putin.

Putin left for the crash site to meet with his Polish counterpart Donald Tusk, who was also enroute to Smolensk.

"The Polish premier wished to travel to the scene of the tragedy. I will go to meet him," Putin said. "We will visit the place of the crash together."

Putin said the bodies of the crash victims would be sent to Moscow for identification, and a special center would be established to help the victims' relatives from Poland.

Shoigu, who was at the site of the crash, said 97 bodies would be placed in coffins and taken to Moscow's Domodedovo airport.

"The bodies of those who died will be delivered to Moscow for identification," he said.

Both "black boxes have been found and examination on them has begun," Shoigu said.